Plot Peak Power et al 1 of 2

Update to let users specify a custom duration
to plot on LTM for peak power, vam, wpk etc.

This update is to the LTM chart settings and
the reading/writing of chart settings.

Next update will update LTMWindow/LTMPlot to
retrieve and plot the values from the CPX files.

Note: the values will not plot yet, the next
      commit in this series is required.