Fix wrapText assignment of dy attribute

[katiepark]

A one-line fix. The wrapText helper function needs to include the initial dy value in addition to any line height offset. It also needs to specify the unit of the value -- can be hard coded to pixels until #248 (handling of units) is closed.

[ghing]

Is this something that works sometimes, or that you have to fix each time you use it?

[katiepark]

It works fine if there's no dy value initially set. I'm just about to push a fix to this that should work in either scenario, though...