imshow: axis not natural

Piotr_Zolnierczuk · February 22, 2008, 9:31pm

I guess it was a FAQ

The correct way - after experimenting with imshow options - is to transpose the data and use origin='lower' option

imshow(c.T,interpolation='nearest',origin='lower')

Piotr

···

----- Original Message ----

From: Piotr Zolnierczuk <zolnie@...9...>

To: matplotlib-users@lists.sourceforge.net

Sent: Friday, February 22, 2008 2:31:08 PM

Subject: [Matplotlib-users] imshow: axis not natural

Hi,

it is probably a faq - but I could not find it anywhere.

I would like to plot a 2D distribution (2D histogram) of some data using a 2D numpy.array that contains my data: c[ix,iy]

When I plot it via imshow the x axis points downwards and the y axis point to the left.

What I want is that the x-axis points to the left and the y axis points upwards.

So I ended up writing a small script twodimshow (below) where I transpose data (x<->y) and also flip the sign of the y-axis

Now: is there a better way of doing it?

See also CERN Root C++ package (ftp://root.cern.ch/root/doc/3Histograms.pdf) and/or (SLAC HIPPO (http://www.slac.stanford.edu/grp/ek/hippodraw/)

------------------- CODE BELOW ---------------------------

import numpy

from pylab import *

def convertToImShow(data,copy=True):

'''Converts data so that imshow displays x,y as a normal

physicist expects ie x pointing left and y pointing up'''

data = numpy.array(data.T,copy=copy)

if data.ndim != 2:

raise TypeError,'data must be a two-dimensional matrix'

nx,ny = data.shape

for i in range(nx/2):

j= nx - i - 1

for k in range(ny): data[j,k],data[i,k]=data[i,k],data[j,k]

return data

def twodimshow(data,extent=None,aspect=None):

r = convertToImShow(data)

if not aspect and extent:

if len(extent)>=4:

aspect = (extent[1] - extent[0])/float(extent[3] - extent[2])

imshow(r,extent=extent,aspect=aspect,interpolation='nearest')

if __name__ == "__main__":

c = zeros((4,4))

for ix in range(4):

for iy in range(ix):

c[ix,iy]=ix+iy

figure()

subplot(221)

imshow(c ,interpolation='nearest')

title('Axis direction not natural')

xlabel('Y-Axis'); ylabel('X-Axis')

subplot(222)

imshow(c.T ,interpolation='nearest')

title('Not there yet')

xlabel('X-Axis'); ylabel('Y-Axis')

subplot(223)

twodimshow(c,extent=(0,4,0,4))

title('That is what I want')

xlabel('X-Axis'); ylabel('Y-Axis')

show()

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