Thread: [Matplotlib-users] Colorbar Ticks

Hello,

I have the problem, that sometimes the first and last ticklabel of a
colorbar is not drawn. E.g. if I create a colorbar through
fig.colorbar(p3,orientation='horizontal',ticks=np.arange(0.0,8,2))
I only get ticklabels at 2,4,6, but I would like it to have 0,2,4,6,8 and I
don't know how to control this behaviour. If I do
cb=fig.colorbar(p3,orientation='horizontal',ticks=np.arange(0.0,8,2))
cb.ax.xaxis.set_ticks(np.arange(0,8,2))
the colorbar vanishes and instead I have 4 ticks with the labels
0.8,1.6,2.4,3.2

Thank you for your help,
Markus


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I should perhaps mention, that if i try
cbar=fig.colorbar(p1,orientation='horizontal',ticks=[0.0,2.0,4.0,6.0,8.0])
cbar.ax.set_xticklabels(['0', '2', '4','6','8'])
the colorbar is drawn correctly, but I get the label 0 at position 2, the
label 2 at position 4 and 4 at pos. 6, the labels at the end are not
drawn...

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marcusantonius wrote:
> I should perhaps mention, that if i try
> cbar=fig.colorbar(p1,orientation='horizontal',ticks=[0.0,2.0,4.0,6.0,8.0])

The ticks that it uses are taken from the list--they are the ones that 
are within the range of numbers mapped to colors.  The list of ticks 
does not set that range.

> cbar.ax.set_xticklabels(['0', '2', '4','6','8'])
Setting ticklabels is dangerous unless you do it based on first getting 
the actual ticks and using their values, or if you are otherwise sure of 
their values.  But it should be very rare that you have to do this.

> the colorbar is drawn correctly, but I get the label 0 at position 2, the
> label 2 at position 4 and 4 at pos. 6, the labels at the end are not
> drawn...

None of the labels correspond to their ticks in this case.

Eric

marcusantonius wrote:
> Hello,
> 
> I have the problem, that sometimes the first and last ticklabel of a
> colorbar is not drawn. E.g. if I create a colorbar through
> fig.colorbar(p3,orientation='horizontal',ticks=np.arange(0.0,8,2))
> I only get ticklabels at 2,4,6, but I would like it to have 0,2,4,6,8 and I
> don't know how to control this behaviour. If I do
> cb=fig.colorbar(p3,orientation='horizontal',ticks=np.arange(0.0,8,2))
> cb.ax.xaxis.set_ticks(np.arange(0,8,2))
> the colorbar vanishes and instead I have 4 ticks with the labels
> 0.8,1.6,2.4,3.2
> 
> Thank you for your help,
> Markus
> 
> 

Markus,

The problem here is not the specification of the ticks, it is the actual 
range of the colorbar, which is taken from the norm object used in the 
color mapping.  Try this (in ipython -pylab):

imshow(rand(10,10)*8, vmin=0, vmax=8)
colorbar(ticks=[0,2,4,6,8])

Alternatively, you can use the set_clim() method on any Mappable such as 
an image, or the pyplot clim() function, to set the limits.

Note also that to get the sequence [0,2,4,6,8] you need arange(0,9,2), 
not arange(0,8,2).  Or you can use linspace(0,8,5) if you prefer.

Eric

Thank you very much for your email.

Your example
>imshow(rand(10,10)*8, vmin=0, vmax=8)
>colorbar(ticks=[0,2,4,6,8])
works fine.

>Note also that to get the sequence [0,2,4,6,8] you need arange(0,9,2), 
>not arange(0,8,2).  Or you can use linspace(0,8,5) if you prefer.
Thank you for making me aware of that (You see that I don't have a lot of
experience in python and matplotlib).

I now specified the color range via vmin and vmax, and this solved my
problem. Thanks for the fast help

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