# How can I draw a circle on a graph using point data and the axis?

G

#### Graphically Challenged

I am working with Mohr circles and I want to draw a circle on my graphs using
a data point (x,y) values to make the circles. Now if I were using a
compass, I would set my compass point on the x-axis at the x-value and then
draw a circle with radius (y) around that point making certain that I
intercept the x-axis at distance (x + y), (x - y) and point (x,y). I am
trying to figure out if there is someway I can conveniently construct these
circles in excel rather than printing out the graph and then hand-drawing
them over the plotted data points.

J

#### Jon Peltier

I haven't done Mohr's Circle since gradual school.... Knowing the (Xc,Yc)
coordinates of the circle's center, and the circle's radius R, you can
calculate points of the circle, then plot the points on the chart: Put Xc,
Yc, and R into B1, C1, and D1.

Enter 0 in A3 and 10 in A4. Select A3:A4, then drag the black square at the
bottom right corner of the selection downward until you have filled from 0
to 360 in steps of 10 (down to A39).

In B3 enter this formula:

=\$B\$1+\$D\$1*COS(A3*PI()/180)

In C3 enter this formula

=\$C\$1+\$D\$1*SIN(A3*PI()/180)

Select B3:C3, and double click on the little black square on the bottom
right corner of the selection, which will fill these formulas down to row
39. Select & copy B3:C39, select the chart, and use Paste Special to add
this data to the chart as a new series.

- Jon