The ‘Window’ property allows for easy programming of ‘switchover’ points in timeseries. Any points outside the given data window (applies to X-axis only) are treated as missing values, so they are used in the auto-ranging of the axes but are not actually plotted. This allows you to change style easily (for example to use dashed lines for the part of the series which is beyond the current month) in the middle of a single timeseries.

© NB - window is 'inclusive' so the boundary point gets hit twice.
 ch.Set('Window' 0 sw)('Line' 'solid')
 ch.Plot mat
 ch.Set('Window'sw 0)('Line' 'Dash')
 ch.Plot mat

You will probably also want to set the markers to a cycle of the same length as the number of columns in mat, so that the same markers are used in each part of the chart.

Note that the ‘Window’ is inclusive, so that the points on the boundary get plotted twice. See Switch 7 in the RainPro workspace for an example.