[Bug] Load arrows from plot_supports_and_loads_2d() not behaving for large model dimensions

[firai]

plot_supports_and_loads_2d() load arrows do not behave as expected when the dimensions of the model are large, which occurs when inch or mm units are used. Specifically, there are two issues:

1. The arrowheads are deformed/practically not visible, regardless of whether the scaling factor sfac is appropriate.
2. The auto-calculated sfac does not seem to be in-scale.

The following model is used an example to illustrate the issues. The member lengths are defined in millimeters, so dimensions are on the order of 10^4. Similar issues exist when inch units are used, which would lead to dimensions on the order of 10^3.

1. Even if I set a manual sfac, the arrowheads are practically invisible:
   
2. If I let the function auto-calculate sfac, then length of the arrows become much larger than the size of the model.
   

However, when I redefine the member lengths in meters, neither of these issues appear.