I need to graph $1 \over d^2$ with average light intensity, where $d$ is $5,10,15,20,25,30$ (i.e. squared would be equal to $1\over25$$1\over100$$1\over225$$1\over400$$1\over625$$1\over900$). The average light intensity would be 766,217,90,50,29,21 respectively (i.e. 766 would be for 5, and 217 would be for 10, etc..)
The graph should look like a constant line just like this but obviously for different x and y variables.
I'm stuck on how to draw the line graph in a way that shows a constant.
On
After a very few attempts but very much guided by your underlying ideas, I effectively found that the reciprocal of the square root of the average light intensity varies linearly with $d$; its is almost perfect (I guess and hope) for your plot requirement.
So, you keep $d$ for the $x$ axis and you change what was $y$ to $\frac{1}{\sqrt{y}}$. The least square regression gives you a slope which is $0.007234$ and no intercept since you did not want any.
Try in http://people.hofstra.edu/Stefan_Waner/newgraph/regressionframes.html with the data:
0.04 766
0.01 217
0.0044 90
0.0025 50
0.0016 29
0.0011 21