plots a 3-D "line" with c-data as color
Makes a surface object and the colouring is controlled by the colormap.

Example:
v_start = [2 1 8]; % m/s
g = 9.81; % m/(s^2)
t_impact = 2*v_start(3)/g; % s
t = [0:t_impact/100:t_impact]'; % s
x = v_start(1)*t; % m
y = v_start(2)*t; % m
z = v_start(3)*t-1/2*g*t.^2; % m
velocity = ([[x(2:numel(t)) y(2:numel(t)) z(2:numel(t))]-[x(1:numel(t)-1) y(1:numel(t)-1) z(1:numel(t)-1)];[0 0 0]]); % m/s
speed = sqrt(sum(velocity.^2,2)); % m/s
color_line3(x,y,z,speed);
axis equal vis3d; view(15,15);
title('trajectory of a projectile, color = speed');
axis equal vis3d; view(15,15);
colorbar