barchart errorbars always in both directions

Community matplotlib-users
1

Hi,

I'm following the example in the gallery to do a barchart plot (see
http://matplotlib.sourceforge.net/examples/api/barchart_demo.html ).

In contrast to the example I would like to see the error bars only above
the bars, so I tried

rects2 = ax.bar(ind+width, womenMeans, width, color='y',
                yerr=stds, error_kw = {'barsabove': True,
                                       'ecolor' : 'k'}

While the 'ecolor' argument gets accepted, 'barsabove' apparently has no
effect (error bars still point up and downwards) - yet, no warning /
error is triggered. Where is my mistake? Or is this a bug (still using
version 1.0.1) with a known work-around?

TIA
Chris

2

Chris,

I don’t think “barsabove” does what you want. By “above”, it means that the errorbar is plotted in a layer on top of the plotting symbol rather than in the layer under it. Both ends will be plotted.

To get what you want, you might want to try (Note: untested):

rects2 = ax.bar(ind+width, womenMeans, width, color=‘y’,

            yerr=np.vstack([[0]*len(stds), stds]), error_kw = {'ecolor'   : 'k'})

When yerr is a 2xN numpy array, errorbars are plotted at y-yerr[0, :] and y+yerr[1,:]. So, np.vstack creates a 2xN array where the first row is all zeros and the second row is the stds values.

I hope that works for you!
Ben Root

···

On Wed, May 23, 2012 at 4:03 AM, Meesters, Aesku.Kipp Institute <meesters@…4092…> wrote:

Hi,

I’m following the example in the gallery to do a barchart plot (see

http://matplotlib.sourceforge.net/examples/api/barchart_demo.html ).

In contrast to the example I would like to see the error bars only above

the bars, so I tried

rects2 = ax.bar(ind+width, womenMeans, width, color=‘y’,

            yerr=stds, error_kw = {'barsabove': True,

                                   'ecolor'   : 'k'}

While the ‘ecolor’ argument gets accepted, ‘barsabove’ apparently has no

effect (error bars still point up and downwards) - yet, no warning /

error is triggered. Where is my mistake? Or is this a bug (still using

version 1.0.1) with a known work-around?

TIA

Chris

3

Thanks, Ben. This is indeed what I was looking for and gives the desired
behavior.

Thanks a lot!
Chris

···

On Wed, 2012-05-23 at 08:55 -0400, Benjamin Root wrote:

On Wed, May 23, 2012 at 4:03 AM, Meesters, Aesku.Kipp Institute > <meesters@...4092...> wrote:
        Hi,
        
        I'm following the example in the gallery to do a barchart plot
        (see
        http://matplotlib.sourceforge.net/examples/api/barchart_demo.html ).
        
        In contrast to the example I would like to see the error bars
        only above
        the bars, so I tried
        
        rects2 = ax.bar(ind+width, womenMeans, width, color='y',
                       yerr=stds, error_kw = {'barsabove': True,
                                              'ecolor' : 'k'}
        
        While the 'ecolor' argument gets accepted, 'barsabove'
        apparently has no
        effect (error bars still point up and downwards) - yet, no
        warning /
        error is triggered. Where is my mistake? Or is this a bug
        (still using
        version 1.0.1) with a known work-around?
        
        TIA
        Chris
        
Chris,

I don't think "barsabove" does what you want. By "above", it means
that the errorbar is plotted in a layer on top of the plotting symbol
rather than in the layer under it. Both ends will be plotted.

To get what you want, you might want to try (Note: untested):

rects2 = ax.bar(ind+width, womenMeans, width, color='y',
               yerr=np.vstack([[0]*len(stds), stds]), error_kw =
{'ecolor' : 'k'})

When yerr is a 2xN numpy array, errorbars are plotted at y-yerr[0, :]
and y+yerr[1,:]. So, np.vstack creates a 2xN array where the first row
is all zeros and the second row is the stds values.

I hope that works for you!
Ben Root