## need help to chart curve on graph with only certain knowns

Hi, hope to get some help on a problem. I have a curve I'm trying to chart, but I only have a certain number of known values, beginning is 0/0 and I don't know the end.

On the left side of the graph is 0 to 100 (more increments the better) and the bottom is 0 to unknown.
The known values of the curve are X - 33.33/ Y - 25, X - 50/Y 33.23, X - 100/Y - 50, X - 200/Y - 66

The maximum for Y is 100 (it's in percent) and the X maximum is unknown.

If you'd like to know what it's for, I need to make a chart to figure out resistance/resistor values for a circuit in series. The Y axis is for % of voltage drop across resistor 1 based on the resistance value of resistor 2 in the series - so, for a voltage drop of 25% across resistor 1, I need to use a resistor with a value of 33.33% of the 2nd resistance value, and so on.

Thanks for any help anyone can give me.
I'm also wondering if there's any free software out there I could get that I can chart this with (being able to go at least 3 decimals would be helpful) once I have the answer? Thanks again.