MATLAB: Plot Line Changes When Adding Patches

datenumdatetickopenglpatchplotprecisionrenderer

Hi, I have a basic plot in MATLAB (plot(x,y)), but when I add patches to the figure (in order to shade regions of night), it changes the original plot line to a step-like line. I am not quite sure why the addition of patches does this. I have two links to images: the first shows the original plot and the second shows what happens to the plot with the addition of patches. Any help would be greatly appreciated. Thank you.
http://dropdo.com/4CT/Fig1
http://dropdo.com/4CV/Fig2

Best Answer

When you add transparency to an object in a figure, the figure will automatically change its renderer to OpenGL. You can read more about MATLAB's renderers here and here.
The thing that's messing you up here is that at some point in the rendering pipeline, OpenGL converts floating point numbers to single precision. When you plot things where the x-data is MATLAB date numbers (e.g. creating using datenum), those numbers are large enough that there is a fairly coarse quantization between them in single precision.
One thing you can try doing to work around the problem is to re-scale the plot's x-data into a more "single-precision-friendly" range. The downside would be that you'd then need to set the tick labels manually. Here's a simple example, adapt as necessary for your data:
x = datenum(2011, 6, 20, 1:12, 0, 0);
y = 1:12;
plot(x,y);
set(gcf, 'Renderer', 'openGL') % Looks bad
% Scale x-data
x2 = x - min(x);
x2 = x2./max(x2);
figure;
plot(x2,y);
set(gcf, 'Renderer', 'openGL') % Looks good
% Set ticks manually
set(gca, 'XTick', x2(1:2:end))
set(gca, 'XTickLabel', datestr(x(1:2:end), 'HH:MM'))
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