Ah, sorry: I misunderstood your initial question
You can write 'Doubles' as Values to a (data)series and by doing so 'add them to a bar's usable data':
Java:
series.setDouble(index, Values.DAY_HIGH, today_high);
Then you could have your code look up the values that you need for your line:
Java:
point1 = series.getDouble(index, Values.DAY_HIGH)
And eventually use those to plot your line.
I have no code for that yet, since I did not need it until now. I guess you could either draw a set of lines between 2 points (and repeat that a number of times, so it will become one longer line) or plot a path over all the points you stored/looked up.
I suggest you look into the SDK's 'standard MW Studies' for a Study that does exactly this and then copy that code.
Let us know if it works or not, and how you implemented it. It might one day serve others