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I am plotting Cumulative Distribution Functions, with a large number of data points. I am plotting a few lines on the same plot, which are identified with markers as it will be printed in black and white. What I would like are markers evenly spaced in the x-dimension. What I am getting is one marker per data point (and given the number of points, they all overlap)

I'm not sure if it's my understanding of how to plot well, or just a lack of understanding matplotlib. I can't find a 'marker frequency' setting.

An easy solution for one line would be to take every N'th value from the line, and use that as a separate line with linestyle='', but I would like the markers to be vertically aligned, and the different x arrays have different lengths. 

# in reality, many thousands of values
x_example = [ 567, 460, 66, 1034, 275, 26, 628, 99, 287, 157, 705, 421, 1093, \ 
     139, 204, 14, 240, 179, 94, 139, 645, 670, 47, 520, 891, 450, 56, 964,   \
     1728, 99, 277, 356, 1628, 745, 364, 88, 112, 810, 816, 523, 401, 89,     \ 
     278, 917, 370, 53, 39, 90, 853, 356 ] 
x = sort(x_example)
y = linspace(0,1,len(x))

ax = subplot(1,1,1)
plots[w] = ax.plot(x,y, marker='o')
James Broadhead
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  • Related solution for arclength marker spacing: http://stackoverflow.com/questions/17406758/plotting-a-curve-with-equidistant-arc-length-markers – Ioannis Filippidis Nov 25 '13 at 08:08

1 Answers1

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You can do plot(x,y,marker='o',markevery=5) to mark every fifth point, but I don't think there is any built-in support for setting marks at even intervals. You could decide on the x locations where you want the marks, use e.g. numpy.searchsorted to find which data points the locations fall between, and then interpolate between the neighboring points to find the y coordinates.

Jouni K. Seppänen
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  • Well, that's certainly nicer than my approach would have been. Time to look up searchsorted! – James Broadhead Jan 12 '10 at 15:06
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    Alternatively, markevery supports a float input which `markers will be spaced at approximately equal distances along the line; the distance along the line between markers is determined by multiplying the display-coordinate distance of the axes bounding-box diagonal by the value` http://matplotlib.org/api/lines_api.html#matplotlib.lines.Line2D.set_markevery – Erik Sep 25 '15 at 19:02