$r=\cos \left(\frac{13}{7}\theta \right)$
When I try I get this: $\left(x^2+y^2\right)^{.5}=\space \cos \left(\frac{13\space }{7\space }\arctan \left(\frac{y}{x}\right)\right)$
But that doesn't seem to graph properly, see: https://www.desmos.com/calculator/vjyhgdcxkz
What am I doing wrong?