**
A three-dimensional plot, and a line of variable color
**

When plotting functions in 3D, two functions need to be given -- one for `y(x),` and a second for `z(x)`. Here, an expression is also given for the color of the line, which varies from point to point. As in expressions passed to the `using` modifier, the columns of data are referred to as `$1` for the first column, i.e. `x`; `$2` for the second column, `y`; etc. The expression given here uses the built-in function `hsb()` to produce a color object with the specified hue, saturation and brightness.

### Script

set xlabel "$x$"
set ylabel "$y$"
set zlabel "$z$"
set xformat r"%s$\pi$"%(x/pi)
set key below
set size 6 ratio 0.5 zratio 0.5
set grid
plot 3d [-10:10][-1:1][-1:1] sin(x):cos(x) with lw 3 \
color hsb($1/20+0.5,0.9,0.8)