### Plot error: a free variable?

This should be a rather simple question, and I think I am just not understanding the error.

I have a function say `f(x,y)= x*y^3*normal(y,1,2)`

where normal() is the normal pdf equation (x,mean,standard deviation).

So I want to integrate across the pdf and then plot x. What I thought I would do was:

```
g = f.integral(y,0,Infinity)
plot(g,-10,10)
```

But that give the error: ValueError: free variable: x

What am I missing?