Fix graph and coloring when ordinal/time scales are used

Adjusts the y-axis and coloring for when ordinal (and as a derivative,
time scales) are the only y-axis in the graph.  Previously, a number-
only axis was required to draw the plot.

Also fixes a bug where selecting ordinal/time scale would not select
the lines.

Closes #54
Merge pull request #57 from polyfractal/master

Fix parallel coordinates when used with ordinal/time scales