Fixed the rendering of block mode to show all data

Block mode rendering is subtley different to normal rendering

In line / smooth mode the starting point is indeed a point, the
last point again is a "point" and the lines are only used to connect
the dots.

In block mode however, the "points" aren't strictly points, they're
a point and the corresponding horizontal line, meaning that you need
to make the graph have more points than you would otherwise