Colorbar labeling

I have a grid with values ranging from exactly 0.0 and 100.0. When I
plot this with colorbar, the base of the colorbar is labeled "-0.0".
Is this a default for 0.0...to plot it with as a negative number? Any
workarounds?

Bruce

···

---------------------------------------
Bruce W. Ford
Clear Science, Inc.

I have a grid with values ranging from exactly 0.0 and 100.0. When I
plot this with colorbar, the base of the colorbar is labeled "-0.0".
Is this a default for 0.0...to plot it with as a negative number? Any
workarounds?

Would you provide a minimal runnable example, please, and specify what mpl version you are using? It is certainly not intended or usual that 0 be displayed as -0.

Eric

···

On 08/20/2010 05:29 AM, Bruce Ford wrote:

Bruce

---------------------------------------
Bruce W. Ford
Clear Science, Inc.

This effect is happening within an web app that displays gridded
fields from multiple datasets (~4500 lines of code). So I it's tricky
to create an example. Although if I use numpy.min(grid) the minimum
is 0. So, I think colorbar or matplotlib is interpreting the 0 as -0.
(Matplotlib version 0.99.0 RC0)

The colorbar call that I'm using is:

cbar = pyplot.colorbar(plot,shrink=0.7, format="%1.1f",
spacing='proportional',orientation='vertical')

cbar.ax.set_ylabel(cbar_label(param,unit))

The function cbar_label is:

def cbar_label(param,unit):
    #Helper function for making colorbar label
    if param == "sig":
        if unit==1:
            cbar_label = "Feet"
        else:
            cbar_label = "Meters"
    elif param == "dir":
        cbar_label = "Radial Direction"
    elif param == "per":
        cbar_label = "Seconds"
    elif param[-5:] == "_wind":
        if unit == 3:
            cbar_label = "Kts"
        else:
            cbar_label = "M/S"
    elif param[-4:] == "_hgt":
        if unit == 5:
            cbar_label = "GPFt"
        else:
            cbar_label = "GPM"
    elif param == "slp":
        cbar_label = "Millibars"
    elif param == "1000_rh":
        cbar_label = "%"
    elif param == "1000_temp":
        if unit == 9:
            cbar_label = "Degrees F"
        else:
            cbar_label = "Degrees C"
    else:
        cbar_label = param
    return cbar_label

If this doesn't offer anything, I'll try to generate a
compartmentalized example of the issue.

Bruce

···

---------------------------------------
Bruce W. Ford
Clear Science, Inc.
bruce@...2905...
http://www.ClearScienceInc.com
http://www.facebook.com/clearscience
http://www.twitter.com/ROVs_rule
Phone: (904) 796-8101
Fax: (904) 379-9704
8241 Parkridge Circle N.
Jacksonville, FL 32211
Skype: bruce.w.ford

---------------------------------------
To schedule a meeting with Bruce, Go to: http://tungle.me/bruceford
----------------------------------------

On Fri, Aug 20, 2010 at 2:06 PM, Eric Firing <efiring@...202...> wrote:

On 08/20/2010 05:29 AM, Bruce Ford wrote:

I have a grid with values ranging from exactly 0.0 and 100.0. When I
plot this with colorbar, the base of the colorbar is labeled "-0.0".
Is this a default for 0.0...to plot it with as a negative number? Any
workarounds?

Would you provide a minimal runnable example, please, and specify what
mpl version you are using? It is certainly not intended or usual that 0
be displayed as -0.

Eric

Bruce

---------------------------------------
Bruce W. Ford
Clear Science, Inc.

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This effect is happening within an web app that displays gridded
fields from multiple datasets (~4500 lines of code). So I it's tricky
to create an example. Although if I use numpy.min(grid) the minimum
is 0. So, I think colorbar or matplotlib is interpreting the 0 as -0.

You are talking about the colorbar tick labels, correct? The lowest tick label is coming out as -0.0?

  (Matplotlib version 0.99.0 RC0)

The colorbar call that I'm using is:

cbar = pyplot.colorbar(plot,shrink=0.7, format="%1.1f",
spacing='proportional',orientation='vertical')

This means your colorbar tick values are simply being formatted by python, like this:

In [1]: "%1.1f" % -0.0000001
Out[1]: '-0.0'

In [2]: "%1.1f" % 0.0000001
Out[2]: '0.0'

In [3]: "%1.1f" % 0.0
Out[3]: '0.0'

In [4]: "%1.1f" % -0.0
Out[4]: '-0.0'

In [5]: import numpy

In [6]: numpy.min(-0.0)
Out[6]: -0

In [7]: -0.0 == 0.0
Out[7]: True

So I suspect the problem is that a small negative value, or a negative zero, is becoming the tick value. I don't know why. You may or may not want to investigate.

I dimly recall a problem like this cropping up on the list before--but I don't remember anything else about it.

Here is a workaround (untested, but should be close):

from matplotlib.ticker import FormatStrFormatter
class MyCBFormatter(FormatStrFormatter):
     def __call__(self, x, pos=None):
         xstr = self.fmt % x
         if float(xstr) == 0:
             return self.fmt % 0
         return xstr
cbar = pyplot.colorbar(plot,shrink=0.7, format=MyCBFormatter("%1.1f"),
          spacing='proportional',orientation='vertical')

Eric

···

On 08/20/2010 10:14 AM, Bruce Ford wrote:

cbar.ax.set_ylabel(cbar_label(param,unit))

The function cbar_label is:

def cbar_label(param,unit):
     #Helper function for making colorbar label
     if param == "sig":
         if unit==1:
             cbar_label = "Feet"
         else:
             cbar_label = "Meters"
     elif param == "dir":
         cbar_label = "Radial Direction"
     elif param == "per":
         cbar_label = "Seconds"
     elif param[-5:] == "_wind":
         if unit == 3:
             cbar_label = "Kts"
         else:
             cbar_label = "M/S"
     elif param[-4:] == "_hgt":
         if unit == 5:
             cbar_label = "GPFt"
         else:
             cbar_label = "GPM"
     elif param == "slp":
         cbar_label = "Millibars"
     elif param == "1000_rh":
         cbar_label = "%"
     elif param == "1000_temp":
         if unit == 9:
             cbar_label = "Degrees F"
         else:
             cbar_label = "Degrees C"
     else:
         cbar_label = param
     return cbar_label

If this doesn't offer anything, I'll try to generate a
compartmentalized example of the issue.

Bruce

Thanks I'll give this a try. numpy.min(grid) reports 0.0 (no
negative) yet it labels as -0.0, BTW, but let me give this a try.

Bruce

···

---------------------------------------
Bruce W. Ford
Clear Science, Inc.
bruce@...2905...
http://www.ClearScienceInc.com
http://www.facebook.com/clearscience
http://www.twitter.com/ROVs_rule
Phone: (904) 796-8101
Fax: (904) 379-9704
8241 Parkridge Circle N.
Jacksonville, FL 32211
Skype: bruce.w.ford

---------------------------------------
To schedule a meeting with Bruce, Go to: http://tungle.me/bruceford
----------------------------------------

On Fri, Aug 20, 2010 at 4:44 PM, Eric Firing <efiring@...202...> wrote:

On 08/20/2010 10:14 AM, Bruce Ford wrote:

This effect is happening within an web app that displays gridded
fields from multiple datasets (~4500 lines of code). So I it's tricky
to create an example. Although if I use numpy.min(grid) the minimum
is 0. So, I think colorbar or matplotlib is interpreting the 0 as -0.

You are talking about the colorbar tick labels, correct? The lowest
tick label is coming out as -0.0?

(Matplotlib version 0.99.0 RC0)

The colorbar call that I'm using is:

cbar = pyplot.colorbar(plot,shrink=0.7, format="%1.1f",
spacing='proportional',orientation='vertical')

This means your colorbar tick values are simply being formatted by
python, like this:

In [1]: "%1.1f" % -0.0000001
Out[1]: '-0.0'

In [2]: "%1.1f" % 0.0000001
Out[2]: '0.0'

In [3]: "%1.1f" % 0.0
Out[3]: '0.0'

In [4]: "%1.1f" % -0.0
Out[4]: '-0.0'

In [5]: import numpy

In [6]: numpy.min(-0.0)
Out[6]: -0

In [7]: -0.0 == 0.0
Out[7]: True

So I suspect the problem is that a small negative value, or a negative
zero, is becoming the tick value. I don't know why. You may or may not
want to investigate.

I dimly recall a problem like this cropping up on the list before--but I
don't remember anything else about it.

Here is a workaround (untested, but should be close):

from matplotlib.ticker import FormatStrFormatter
class MyCBFormatter(FormatStrFormatter):
def __call__(self, x, pos=None):
xstr = self.fmt % x
if float(xstr) == 0:
return self.fmt % 0
return xstr
cbar = pyplot.colorbar(plot,shrink=0.7, format=MyCBFormatter("%1.1f"),
spacing='proportional',orientation='vertical')

Eric

cbar.ax.set_ylabel(cbar_label(param,unit))

The function cbar_label is:

def cbar_label(param,unit):
#Helper function for making colorbar label
if param == "sig":
if unit==1:
cbar_label = "Feet"
else:
cbar_label = "Meters"
elif param == "dir":
cbar_label = "Radial Direction"
elif param == "per":
cbar_label = "Seconds"
elif param[-5:] == "_wind":
if unit == 3:
cbar_label = "Kts"
else:
cbar_label = "M/S"
elif param[-4:] == "_hgt":
if unit == 5:
cbar_label = "GPFt"
else:
cbar_label = "GPM"
elif param == "slp":
cbar_label = "Millibars"
elif param == "1000_rh":
cbar_label = "%"
elif param == "1000_temp":
if unit == 9:
cbar_label = "Degrees F"
else:
cbar_label = "Degrees C"
else:
cbar_label = param
return cbar_label

If this doesn't offer anything, I'll try to generate a
compartmentalized example of the issue.

Bruce

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Thanks I'll give this a try. numpy.min(grid) reports 0.0 (no
negative) yet it labels as -0.0, BTW, but let me give this a try.

Bruce,

What matters is not min(grid), but the value of the tick. Unless you are forcing them to be the same via a kwarg (which I don't see), they may differ, as seems to be the case. Unfortunately, this is hard to debug, because there is no way to get at the set of numbers that are being formatted to give the tick labels. Doing something like this:

print cbar.cax.get_yticks()

will be of no help, because the tick positions are on a 0-1 scale regardless of the values they represent.

You can, however, specify the desired tick values as a sequence via the ticks kwarg to colorbar. Ticks in that sequence but outside the actual colorbar range (as set by the clim() function, for example) will not appear.

Eric

···

On 08/20/2010 10:51 AM, Bruce Ford wrote:

Bruce

---------------------------------------
Bruce W. Ford
Clear Science, Inc.
bruce@...2905...
http://www.ClearScienceInc.com
http://www.facebook.com/clearscience
http://www.twitter.com/ROVs_rule
Phone: (904) 796-8101
Fax: (904) 379-9704
8241 Parkridge Circle N.
Jacksonville, FL 32211
Skype: bruce.w.ford

---------------------------------------
To schedule a meeting with Bruce, Go to: http://tungle.me/bruceford
----------------------------------------

On Fri, Aug 20, 2010 at 4:44 PM, Eric Firing<efiring@...202...> wrote:

On 08/20/2010 10:14 AM, Bruce Ford wrote:

This effect is happening within an web app that displays gridded
fields from multiple datasets (~4500 lines of code). So I it's tricky
to create an example. Although if I use numpy.min(grid) the minimum
is 0. So, I think colorbar or matplotlib is interpreting the 0 as -0.

You are talking about the colorbar tick labels, correct? The lowest
tick label is coming out as -0.0?

   (Matplotlib version 0.99.0 RC0)

The colorbar call that I'm using is:

cbar = pyplot.colorbar(plot,shrink=0.7, format="%1.1f",
spacing='proportional',orientation='vertical')

This means your colorbar tick values are simply being formatted by
python, like this:

In [1]: "%1.1f" % -0.0000001
Out[1]: '-0.0'

In [2]: "%1.1f" % 0.0000001
Out[2]: '0.0'

In [3]: "%1.1f" % 0.0
Out[3]: '0.0'

In [4]: "%1.1f" % -0.0
Out[4]: '-0.0'

In [5]: import numpy

In [6]: numpy.min(-0.0)
Out[6]: -0

In [7]: -0.0 == 0.0
Out[7]: True

So I suspect the problem is that a small negative value, or a negative
zero, is becoming the tick value. I don't know why. You may or may not
want to investigate.

I dimly recall a problem like this cropping up on the list before--but I
don't remember anything else about it.

Here is a workaround (untested, but should be close):

from matplotlib.ticker import FormatStrFormatter
class MyCBFormatter(FormatStrFormatter):
     def __call__(self, x, pos=None):
         xstr = self.fmt % x
         if float(xstr) == 0:
             return self.fmt % 0
         return xstr
cbar = pyplot.colorbar(plot,shrink=0.7, format=MyCBFormatter("%1.1f"),
          spacing='proportional',orientation='vertical')

Eric

cbar.ax.set_ylabel(cbar_label(param,unit))

The function cbar_label is:

def cbar_label(param,unit):
      #Helper function for making colorbar label
      if param == "sig":
          if unit==1:
              cbar_label = "Feet"
          else:
              cbar_label = "Meters"
      elif param == "dir":
          cbar_label = "Radial Direction"
      elif param == "per":
          cbar_label = "Seconds"
      elif param[-5:] == "_wind":
          if unit == 3:
              cbar_label = "Kts"
          else:
              cbar_label = "M/S"
      elif param[-4:] == "_hgt":
          if unit == 5:
              cbar_label = "GPFt"
          else:
              cbar_label = "GPM"
      elif param == "slp":
          cbar_label = "Millibars"
      elif param == "1000_rh":
          cbar_label = "%"
      elif param == "1000_temp":
          if unit == 9:
              cbar_label = "Degrees F"
          else:
              cbar_label = "Degrees C"
      else:
          cbar_label = param
      return cbar_label

If this doesn't offer anything, I'll try to generate a
compartmentalized example of the issue.

Bruce

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2010/8/20 Bruce Ford <bruce@...2905...>:

I have a grid with values ranging from exactly 0.0 and 100.0. When I
plot this with colorbar, the base of the colorbar is labeled "-0.0".
Is this a default for 0.0...to plot it with as a negative number? Any
workarounds?

How sure are you that the floating point number underneath has really
not set the negative bit?

Maybe try to .clip() the grid positions before on the numpy level to
(0.0, \infty) where 0 arises from a fresh parse of '0.0' by the
interpreter, i.e., a "really really zero" fp number.

To clip, you may also use

grid *= (grid > 0)

or similar things of that fashion.

I'm not sure if the mistake is implied my your side of the code, but i
just hope this helps maybe. The Locator stuff is not soo easy.

Friedrich