Changed bottom to y1

Corrects an Issue found to only occur in Chrome for Windows 7 and Android, as reported in:
https://github.com/flot/flot/issues/1352

By having the coordinate set to bottom, the chart would plot an area starting from the bottom of the charting area, whereas what we actually want is for the chart to start plotting this area from the points of the first coordinate (x1, y1).

tl;dr: We want to plot from (x1,y1) instead of (x1,bottom).