Create an XY Graph and feed it a time stamped XY plot with some hundred thousand points...and you have yourself a very sluggish and possibly crash-ready application. The regular graph can take a bit more data, but still has its limits. Having 100k number of points to display is quite common (in my case it's most often months of 1 second data).
The idea could be formulated to just "improve how graphs handle large data sets"...but how that would be done depends a bit on what optimizations the graph code is open for. The most effective solution however would probably be to do what you currently have to write yourself - a surrounding decimation logic.
So my suggestion is to add an in-built decimation feature, where you can choose to have it automatically operated when needed - or when you say it is needed, and possibly with a few different ways to do the decimation (Min-Max/ Nth point etc.). The automatics should be on by default - making the problem virtually invisible for the novice user.
A big advantage of doing it within the graph is that it will (should) integrate fully with the other features of the graph - like zooming, cursors etc.
