How do I correctly scale my linear graph?

Question

Hi,
I'm creating a simple application for linear graph plotting, just for learning and fun. At the moment it works pretty much as desired except for one thing: if the x and y axis have different scaling, the graph will be incorrect.
A picture should clarify what I mean: [http://i33.tinypic.com/bhzp89.png]
So basically I suppose I need to find a way for my program to adjust the gradient depending on like, distance between the help lines?
This is my little method which returns the y coordinate based on the given x coordinate:
private int calculateY(int x) {
    int y = (int) (getHeight() - (m*x + b*yLineDist));
    y -= getHeight()/2;
    return y;
}Pretty straightforward; the familiar linear equation. I have to do everything "backwards" because the y axis in Java is inverted compared to the standard "mathematical" one, but I guess you can see that. yLineDist would be the distance between two help lines (or what you should call them). There also exists a corresponding xLineDist.
Any ideas? Thanks in advance.

Answer 1

In general, you want to try to keep your model and display elements separate if you can. For your program, the linear equation is the model, and the graph is your display. Both have x and y elements, and I would probably keep them separate for clarity. modelY = m * modelX + b; then for any modelX and modelY, you have a scale to covert those to displayX and displayY. Your particular example seems to be taking in a modelX and returning a displayY which seems a bit confusing to me.
The main reason for doing it like this is so if you ever want to reuse or update old code, you know what your variables represent. Without looking at the code, you wouldn't know which kind of x and y your calculateY method takes and returns.