[Matplotlib-users] Histogram and changing x-axis limits

[Engelsma, D. <D.E...@La...>]

Hi -

I'm using the following code to generate a histogram. I also want to plot
the Upper Control Limit as a vertical line to the right of the histogram
bars and a Lower Control Limit to the left of the bars. Each Control Limit
would be represented by a vertical line. The code below does not include the
line necessary to plot a sample control limit.

# the histogram of the data
n, bins, patches = matplotlib.matlab.hist(histogram_data, 10, normed=0)
# add a 'best fit' line
y = matplotlib.matlab.normpdf(bins, average[i], std_dev[i])
l = matplotlib.matlab.plot(bins, y, 'r--')
matplotlib.matlab.set(l, 'linewidth', 2)
matplotlib.matlab.set(matplotlib.matlab.gca(), 'xlim', [199, 205])
matplotlib.matlab.xlabel(DataDescription[i])
matplotlib.matlab.ylabel('Number of Parts')
matplotlib.matlab.title(PartNum + " -- " + PartDesc)
matplotlib.matlab.show()

When I modify the code with the additional line to plot a Lower Control
Limit: 

# the histogram of the data
n, bins, patches = matplotlib.matlab.hist(histogram_data, 10, normed=0)
# add a 'best fit' line
y = matplotlib.matlab.normpdf(bins, average[i], std_dev[i])
l = matplotlib.matlab.plot(bins, y, 'r--')
matplotlib.matlab.set(l, 'linewidth', 2)
matplotlib.matlab.set(matplotlib.matlab.gca(), 'xlim', [199, 205])
matplotlib.matlab.xlabel(DataDescription[i])
matplotlib.matlab.ylabel('Number of Parts')
matplotlib.matlab.plot([201,201],[0,40],'k--')	<--- added this line to plot
a control limit
matplotlib.matlab.title(PartNum + " -- " + PartDesc)
matplotlib.matlab.show()

The x-axis limits are somehow automatically forced to something less than
that specified in the line: matplotlib.matlab.set(matplotlib.matlab.gca(),
'xlim', [199, 205]). The histogram appears as would be expected, but now I
cannot see my Control Limit (specified in the added line of code) as it
seems to be plotted off of the chart.

If I comment out the additional line (that plotted a vertical line as a
control limit), the x-axis limits return to normal, as expected (199 ->
205).

Why are the x-axis limits adjusted when I try to plot a vertical line as a
Control Limit?

Thanks in advance,

Dave Engelsma