This was looked into a couple of years ago, whilst I was developing the original plugin. Adding support for 'touch' events was easy and I could get in drawing lines with only a couple of lines of code change.
The problem was, however, that the only touchscreen device I have to test this on was my phone. The phone could only display a small portion of the PDF image at a time. If you were using 'touch+drag' to draw lines, then you no longer had any way of scrolling around the page - you could build up a bit of momentum and scroll to the middle of the page, but once you were there you couldn't get back to the rest of the page, as any attempts to 'drag' the page resulted in new annotations being added.
As I never had the spare time to come up with a satisfactory solution to this problem, touch support was left out of the plugin (I thought maybe a floating 'home' button to scroll you back to the top of the page, or possible a 'scroll' icon that would allow you to move the page around when dragged, or scroll-bars, or a floating toolbar).