This jsfiddle shows a couple of the problems:
I have to tap the legend several times on my phone to get it to register. Making the font larger only helps marginally.
I cannot scroll by swiping up/down on the chart. This is a problem if the chart is taking up a large portion of the phone screen. Can it be set to ignore up/down movement and let it pass through?
The legend square/circles thing was a misunderstanding. The legend shows squares when it’s set to “stepArea”, even if the markers are not showing up on the chart.
As far as the viewportMinimum, the problem is that my readings are every minute, on the minute (no seconds). When you zoom it, it is extrapolating the value of where the line goes off the left edge of the screen and setting viewportMinimum with seconds. So the last displayed reading may be 11:43:00am but viewportMinimum is set to 11:42:37am. I need to know the value of the last displayed reading.
I realize that using interval with dates is not needed, but I assume you’d not want it to lock up. It was just fyi.