I'm a total newbie to both linux and C++

I do not get the Draw part of PlotCurve. I'm supposed to use it when I want to incementally update one small part of the curve.

For example, I want to draw FROM X= 67, Y=4.40 TO X=68,Y=4.49
How can I do that using draw, when draw only accept int from and int to? How can one int store the location of a point in a two dimensional graph?

I'm missing something fundamental here but what

Brg
Jore