add --xhline and --yhline options and arbitrary --markup

[o-smirnov]

Some plots could benefit with lines at e.g. 0 to guide the eye.

[o-smirnov]

Had an even better idea -- it's pretty simple to make a generic thingy for invoking arbitrary matplotlib Axes methods. Just supply the arguments as a YaML string. For example, to call Axes.annotate():

shadems msdir/MS-pre_cal.ms -x real -y imag -c CORR --corr IQUV --dir plots --col CORRECTED_DATA --field J0521+1638 --vline 0: --hline 0: -M annotate "['This line is for Paolo', {xy: [0.2, 0], xytext: [0.4, -0.1], arrowprops: {facecolor: black, shrink: 0.05}}]"

[IanHeywood]

That's brilliant.