Rainpro will always range the axes for you on the first plotting call, unless you have already set the ranges explicitly. If you have a matrix of data, and want to use different plotting styles for various columns, it can be a problem if the first subset you plot does not span the full range of the data. This command takes the entire matrix and does the automatic ranging for you, without actually plotting any values.

chRangeAxes mat
chPlot mat[;1]
chVline mat[;2 3]

This will draw the Y-axis to accommodate all the data, even though the first chart plotted only uses the mean values of the daily range plotted by the line following. It is also useful in trellis plots where you typically want all the miniature charts ranged the same so you can compare them easily.

See also ...
Trellis – trellis of small charts
XRange – X-axis range
YRange – Y-axis range