Why does plot3 only show axes in GNU Octave?

Question

We want to make a 3D plot of values that are calculated by GNU Octave. It only shows us the axes of the graph, no dots. When making a normal 2D plot (i.e. plot (x,y)) everything works normally. The code we used was:

t=0;
dt = 0.1;

x=0;
y=0;
z=0;
vx=40;
vy=15;
vz=15;

m=0.43;
g=9.81;
rho=0.9982;
Cw=0.47;
A=0.03789;
                               


while (z>=0)
                               
  Fwlx = 0.5*Cw*A*rho*(vx)^2;
  Fwly = 0.5*Cw*A*rho*(vy)^2;
  Fwlz = 0.5*Cw*A*rho*(vz)^2;

  Fz = m*g;

  Fresx = -Fwlx;
  Fresy = -Fwly;
  Fresz = -Fwlz-Fz;

  ax = Fresx/m;
  ay = Fresy/m;
  az = Fresz/m;

  vx = vx + ax*dt;
  vy = vy + ay*dt;
  vz = vz + az*dt;

  x = x + vx*dt;
  y = y + vy*dt;
  z = z + vz*dt;

  t = t + dt;


  plot3 (x,y,z)
  hold on


end

Answer 1

EDIT 15-Oct-2024: A change has been pushed to Octave's plot3 code that will have it behave similarly to plot, creating a graph with a single data point and the markertype set to .. This will take effect in Octave 10.

This is (unexpectedly) due to the way you are collecting and plotting data 1 point at a time. At least in Octave 9.2.0, plot() defaults to having a point marker for 1 element plots, plot3() has marker type set to 'none'

If you collect the data points and plot them all at once, it links the data points with lines and you can see the trajectory. Alternatively, setting a plot/marker style can make the original approach more visible. E.g.,:

simply adding a format to the plot3 command:

`plot3(x,y,z, 'r*')

shows me the following:

or, if I change the script to something like the following, it will plot all of the points together after the loop:

t=0;
dt = 0.1;

x=0;
y=0;
z=0;
vx=40;
vy=15;
vz=15;

m=0.43;
g=9.81;
rho=0.9982;
Cw=0.47;
A=0.03789;

xp = yp = zp = [];

while (z>=0)

  Fwlx = 0.5*Cw*A*rho*(vx)^2;
  Fwly = 0.5*Cw*A*rho*(vy)^2;
  Fwlz = 0.5*Cw*A*rho*(vz)^2;

  Fz = m*g;

  Fresx = -Fwlx;
  Fresy = -Fwly;
  Fresz = -Fwlz-Fz;

  ax = Fresx/m;
  ay = Fresy/m;
  az = Fresz/m;

  vx = vx + ax*dt;
  vy = vy + ay*dt;
  vz = vz + az*dt;

  x = x + vx*dt;
  y = y + vy*dt;
  z = z + vz*dt;

  t = t + dt;

  xp(end+1) = x;
  yp(end+1) = y;
  zp(end+1) = z;
end
  plot3 (xp,yp, zp)

producing:

regarding whether or note this might be a bug, note that a simple 1 data point plot with clearly visible data:

    A = plot(1,1)

    get(A)

    ...
    linestyle = -
    linewidth = 0.5000
    marker = .
    markeredgecolor = auto
    markerfacecolor = none
    markersize = 6
    ...

but with a 3d plot:

    plot3(1,1,1)

    get(A)
    ...
    linestyle = -
    linewidth = 0.5000
    marker = none
    markeredgecolor = auto
    markerfacecolor = none
    markersize = 6
    ...

Note the marker = none setting for the 3D plot. plot() has marker = none for data with more than one point, so I suspect there was a conscious decision to changes this behavior for n = 1. plot3() does not do this.

It is worth noting that while Octave is designed with intent to be m-code compatible with Matlab, the plot and plot3 in Matlab both behave like plot3 in Octave, where a plot with n=1 is treated no differently than one with n>1. They have the default properties for a line plot, with a single solid line and no marker points.