Bug: Caption in historical charts with multiple days per pixel changes with chart size

Historical chart generation with multiple days per pixel could report different values for the trend analysis and flag fraction in the caption depending upon the chart size (and hence the number of days aggregated into each horizontal pixel). This was because {\tt getDays} is not guaranteed to examine every day in the interval, particularly in the case of long intervals and small charts. I added code to {\tt drawChart} in {\tt history.pm} to call {\tt analyseTrend} for the entire interval to perform the analysis and use the values it computes for the caption, instead of those computed on the fly by {\tt getDays} which may depend upon the chart scale. (Reported by Jim Hollcraft.)