plot color as a function of values?

Hi,

Imagine you have something like:

from pylab import *
t = arange(0.0, 2.0, 0.01)
s = sin(2*pi*t)
ax = subplot(111)
ax.plot(t, s)

That's fine but now I would like to plot the negative parts of the curve in red and the positive one in green.
Is there a nice pylab oriented way to do that? Some kind of "conditional formating"?

Xavier

Xavier Gnata wrote:

Hi,

Imagine you have something like:

from pylab import *
t = arange(0.0, 2.0, 0.01)
s = sin(2*pi*t)
ax = subplot(111)
ax.plot(t, s)

That's fine but now I would like to plot the negative parts of the curve in red and the positive one in green.
Is there a nice pylab oriented way to do that? Some kind of "conditional formating"?

Not built in, but you can do it easily with masked arrays. See http://matplotlib.sourceforge.net/examples/pylab_examples/masked_demo.html

It is not exactly what you want, but close:

sneg = np.ma.masked_greater_equal(s, 0)
spos = np.ma.masked_less_equal(s, 0)
ax.plot(t, spos, 'g')
ax.plot(t, sneg, 'r')

What this does not do is ensure that there is no gap where the line crosses zero. For that, you would need to ensure that your sampling of s(t) includes the zeros.

Eric

···

Xavier

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Eric Firing wrote:

Xavier Gnata wrote:

Hi,

Imagine you have something like:

from pylab import *
t = arange(0.0, 2.0, 0.01)
s = sin(2*pi*t)
ax = subplot(111)
ax.plot(t, s)

That's fine but now I would like to plot the negative parts of the curve in red and the positive one in green.
Is there a nice pylab oriented way to do that? Some kind of "conditional formating"?

Not built in, but you can do it easily with masked arrays. See http://matplotlib.sourceforge.net/examples/pylab_examples/masked_demo.html

It is not exactly what you want, but close:

sneg = np.ma.masked_greater_equal(s, 0)
spos = np.ma.masked_less_equal(s, 0)
ax.plot(t, spos, 'g')
ax.plot(t, sneg, 'r')

What this does not do is ensure that there is no gap where the line crosses zero. For that, you would need to ensure that your sampling of s(t) includes the zeros.

Eric

It should do the trick because my sampling is very high.

Xavier

Maybe a little shorter is the where() keyword, and even that can be omitted:

ax.plot(t[where(s>=0)],s[where(s>=0)],"g")
ax.plot(t[where(s<0)],s[where(s<0)],"r")

or, shorter:

ax.plot(t[s>=0],s[s>=0],"g")
ax.plot(t[s<0],s[s<0],"r")

cheers

Thomas

Xavier Gnata-2 wrote:

···

Hi,

Imagine you have something like:

from pylab import *
t = arange(0.0, 2.0, 0.01)
s = sin(2*pi*t)
ax = subplot(111)
ax.plot(t, s)

That's fine but now I would like to plot the negative parts of the curve
in red and the positive one in green.
Is there a nice pylab oriented way to do that? Some kind of "conditional
formating"?

Xavier

------------------------------------------------------------------------------
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developing skills, take BlackBerry mobile applications to market and stay
ahead of the curve. Join us from November 9 - 12, 2009. Register now!
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View this message in context: http://www.nabble.com/plot-color-as-a-function-of-values--tp25848622p25858967.html
Sent from the matplotlib - users mailing list archive at Nabble.com.

ax.plot(t[s>=0],s[s>=0],"g")
ax.plot(t[s<0],s[s<0],"r")

Whaou! That's what I call a nice pythonic syntax.

XAvier

···

Maybe a little shorter is the where() keyword, and even that can be omitted:

ax.plot(t[where(s>=0)],s[where(s>=0)],"g")
ax.plot(t[where(s<0)],s[where(s<0)],"r")

or, shorter:

ax.plot(t[s>=0],s[s>=0],"g")
ax.plot(t[s<0],s[s<0],"r")

cheers

Thomas

Xavier Gnata-2 wrote:
  

Hi,

Imagine you have something like:

from pylab import *
t = arange(0.0, 2.0, 0.01)
s = sin(2*pi*t)
ax = subplot(111)
ax.plot(t, s)

That's fine but now I would like to plot the negative parts of the curve in red and the positive one in green.
Is there a nice pylab oriented way to do that? Some kind of "conditional formating"?

Xavier

------------------------------------------------------------------------------
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is the only developer event you need to attend this year. Jumpstart your
developing skills, take BlackBerry mobile applications to market and stay ahead of the curve. Join us from November 9 - 12, 2009. Register now!
http://p.sf.net/sfu/devconference
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Xavier Gnata wrote:

ax.plot(t[s>=0],s[s>=0],"g")
ax.plot(t[s<0],s[s<0],"r")

I don't think it does what you want, though, unless you are plotting markers, not lines. With lines, you will have line segments approximately on the x-axis across the gaps. Using masked arrays avoids that. The gaps (masked segments) will not be stroked.

Eric

···

Whaou! That's what I call a nice pythonic syntax.

XAvier

Maybe a little shorter is the where() keyword, and even that can be omitted:

ax.plot(t[where(s>=0)],s[where(s>=0)],"g")
ax.plot(t[where(s<0)],s[where(s<0)],"r")

or, shorter:

ax.plot(t[s>=0],s[s>=0],"g")
ax.plot(t[s<0],s[s<0],"r")

cheers

Thomas

Xavier Gnata-2 wrote:
  

Hi,

Imagine you have something like:

from pylab import *
t = arange(0.0, 2.0, 0.01)
s = sin(2*pi*t)
ax = subplot(111)
ax.plot(t, s)

That's fine but now I would like to plot the negative parts of the curve in red and the positive one in green.
Is there a nice pylab oriented way to do that? Some kind of "conditional formating"?

Xavier

------------------------------------------------------------------------------
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is the only developer event you need to attend this year. Jumpstart your
developing skills, take BlackBerry mobile applications to market and stay ahead of the curve. Join us from November 9 - 12, 2009. Register now!
http://p.sf.net/sfu/devconference
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