Draw bars using fillRect instead of paths.

This is up to 2x faster and appears to work around issues in Chrome's
canvas implementation that sometimes result in bars not being filled.
Resolves #915.
Remove the unused offset parameter.

The drawBar method was always called with an offset of zero, and I see
no other way in which it is currently used. Resolves #382.
Fixed autoscale check when using null x/y values.

The autoscale check was too broad; it included the case where autoscale
was undefined.  This resulted in axes not expanding correctly when
coordinates at the end of a series had null x or y values.  Fixed by
narrowing the check to !== false; resolves #1095.