Re: [Matplotlib-users] strange behaviour with fill_between

[Eric F. <ef...@ha...>]

On 2012/08/27 5:10 AM, Francesco Montesano wrote:
> Dear matplotlibers,
>
> I encountered a bug (?) in fill_between when using logarithmic scales and
> the last part of y and yerr arrays as set to zero: a diagonal stripe going from
> the rightmost non zero value to the first value is drawn.
> It's visible in the right panel of the attached figure, while is not
> present if the plot is linear (left panel).
> If xaxis is log and yaxis is linear the plot is correctly drawn.
>
> I'm using mpl.__version__ = '1.1.1rc' under Kubuntu 12.04 with Python 2.7.3
>
> The plot has been created with the script below.
>
> Is this a bug or am I missing something?

I don't think it is exactly a bug, but I don't know why the fill region 
is appearing as it does.  The underlying problem is that fill_between is 
doing what it is told to do without knowing that it is going to be 
plotted on a log axis.

A good workaround is to change your call to fill_between to look like this:
positive = y - yerr > 0
ax2.fill_between( x,y-yerr,y+yerr, where=positive, color='b', alpha=0.4)

Alternatively, you could use np.clip to put a floor under y - yerr and y 
+ yerr.

Eric

>
> Cheers
> Francesco
>
>
> ##### error_fill_between.py ######
> import matplotlib.pyplot as plt
> import numpy as np
>
> #values to plot
> x = np.linspace( 1, 10, num=100 )
> y = np.exp( -x**2 )
> y[50:] = 0
> yerr = y* np.random.rand(100)
>
> #figure
> fig = plt.figure()
>
> ax1 = fig.add_subplot(121)  #first axes: linear
> ax1.errorbar( x,y,yerr, c='r' )
> ax1.fill_between( x,y-yerr,y+yerr, color='b', alpha=0.4 )
>
> ax2 = fig.add_subplot(122)  #second axes: logarithmic
> ax2.errorbar( x,y,yerr, c='r' )
> ax2.fill_between( x,y-yerr,y+yerr, color='b', alpha=0.4 )
> ax2.set_xscale( "log" )
> ax2.set_yscale( "log" )
>
> plt.show()
> ###### end script  #########
>
>
>
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