Bug: Diet plan outside date range of historical chart causes scaling error

If the diet plan was outside the date range plotted in an historical chart and consequently reported $-1$ for the plan weight, it was still used to scale the weight axis in historical charts, squashing all of the weight entries toward the top of the chart. I modified the automatic scaling in {\tt history::drawChart} to ignore these out of domain values and only include diet calculator values in the scale computation if they actually contribute data to be plotted in the chart. (Reported by Lorenzo Emilitri.)