intermediate complex expression in real functions make many plot functions fail

[jasongrout]

All of the following plots fail

x, y = SR.var('x y')
contour_plot(abs(x+i*y), (x,-1,1), (y,-1,1))
density_plot(abs(x+i*y), (x,-1,1), (y,-1,1))
plot3d(abs(x+i*y), (x,-1,1),(y,-1,1))
streamline_plot(abs(x+i*y), (x,-1,1),(y,-1,1))

with

TypeError: unable to coerce to a real number

The culprit is the call to setup_for_eval_on_grid (from sage/plot/misc.py) that tries to compile the symbolic expression with fast_float. But since the expression involves an intermediate complex number the compilation fails. This can be tested with any of the two

fast_float(abs(x + i*y), x, y)
fast_callable(abs(x + i*y), vars=[x,y])

The function compilation succeeds if we ask for a complex function instead

fast_callable(abs(x + i*y), vars=[x,y], domain=complex)

See also this question on ask.sagemath.org.

CC: @orlitzky @egourgoulhon @JungMath

Component: graphics

Author: Michael Orlitzky

Branch: 6531609

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/8450

[jasongrout]

comment:1

#5572 will help solve this.

[videlec]

Description changed:

--- 
+++ 
@@ -17,3 +17,5 @@
 ```
 contour_plot(imag(log(x+y*I+.001)), (x,-3,3),(y,-3,3),fill=False)
 ```
+
+See also [this question on ask.sagemath.org](https://ask.sagemath.org/question/46275/typeerror-unable-to-coerce-to-a-real-number/).