This function is a minimal wrapper for the matlab plot function with automatic downsampling at low zoom factors and cropping at high zoom factors for faster zooms and pans.
It works in subplots and can be combined with normals plots of any kind in the same axes.
The amount of downsampling can be changed by adjusting the value of MAXPOINTS.
Note that jplot uses the 'tag' and 'userdata' properties of the plotted lines.
Takes initial xL as [min(x) max(x)] rather than axes xlims so that downsampling works correctly if you're zoomed into an existing axis when you jplot.