+ // For stacked graphs, track the baseline for filling.
+ //
+ // The filled areas below graph lines are trapezoids with two
+ // vertical edges. The top edge is the line segment being drawn, and
+ // the baseline is the bottom edge. Each baseline corresponds to the
+ // top line segment from the previous stacked line. In the case of
+ // step plots, the trapezoids are rectangles.
+ var baseline = {};