correct tickGenerator calculation edge case.

During real-time plot updates, if the following option is enabled:
  "xaxis: {show: true}"

then the original calculation in tickGenerator() would draw the right
edge of the chart in two different locations. The final tick at
(axis.max%tickSize==0) is omitted, and the right edge was drawn slightly
to the left. This resulted in jumpy display on rapid updates.

This patch changes the less-than condition to less-than-or-equal. As a
result, the right edge is drawn in a consistent location. And,
tickGenerator() returns a tick when (axis.max%tickSize==0).