Look at the variable ax.
>> fig = figure;
ax = fig.CurrentAxes
ax =
0×0 empty GraphicsPlaceholder array.
That's not an axes. When you create a figure, it doesn't have an axes on it unless you've specifically set up a CreateFcn or something similar that adds an axes. [When you call a plotting function like plot it creates the figure then creates an axes on that figure and finally creates the plot on the axes.] Compare with:
>> fig = figure;
axes(fig);
ax = fig.CurrentAxes
ax =
Axes with properties:
...
The error message could probably stand to be a bit more descriptive. If you feel strongly about that, I recommend submitting that to Technical Support as an enhancement request using the Contact Us link in the upper-right corner of this page.
