YXTOXY - Compute a regularly-sampled, monotonically increasing function x(y)
	from a regularly-sampled, monotonically increasing function y(x) by
	inverse linear interpolation.

yxtoxy		compute a regularly sampled function x(y) from a regularly
		sampled, monotonically increasing function y(x)

Function Prototype:
void yxtoxy (int nx, float dx, float fx, float y[], 
	int ny, float dy, float fy, float xylo, float xyhi, float x[]);

Input:
nx		number of samples of y(x)
dx		x sampling interval; dx>0.0 is required
fx		first x
y		array[nx] of y(x) values; y[0] < y[1] < ... < y[nx-1] required
ny		number of samples of x(y)
dy		y sampling interval; dy>0.0 is required
fy		first y
xylo		x value assigned to x(y) when y is less than smallest y(x)
xyhi		x value assigned to x(y) when y is greater than largest y(x)

Output:
x		array[ny] of x(y) values

Notes:
User must ensure that:
(1) dx>0.0 && dy>0.0
(2) y[0] < y[1] < ... < y[nx-1]

Author:  Dave Hale, Colorado School of Mines, 06/02/89