Calculating relative point coordinates for a graph?

I'm making a simple JavaScript graphing library using the canvas element. I really suck at math so I'm stuck with a simple issue.

If I have a number - for example 30000, and I want to plot it relatively to graph's height which is 4开发者_运维知识库00. How do I calculate the y value for that?

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You would want to figure out your max for the graph. Say, in this case 50000. Then, take your height and divide it by the max (so 400/50000) to get a ratio multiplier. Any number you want to plot you multiply by that ratio and that should give you a number that fits on your space. Is that what you're asking for?

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For that you need to first find out the maximum and minimum values that you need to plot. For example, in this list of (x,y) coordinates: [(1,3),(2,10),(3,0),(4,-10)], the max value is 10 and the min value is -10. This gives you a span of (max-min = 10-(-10) = ) 20.

Notice that you can now translate the set of y values into a number in the range [0,max-min] (i.e. [0,20] in this case). Here, a value of 0 will get plotted as a 0 in the graph and a value of 20 will get plotted as 400. Also, a value of 20/2 will be plotted as 400/2. Thus, a value of 20/x is plotted as 400/x. This means that any value can now be plotted as 400*value/20.

So, to translate a given value n to its corresponding y value on the graph, simply convert n to (n-min)*400/(max-min).