[Matplotlib-users] imshow: axis not natural

[Piotr Z. <zo...@ya...>]

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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