Remove panic on plot grid

[nrot]

Close issue.

[emilk]

Suggested change

	if step_size.abs() == 0.0 {
	trace!("To small step_size");
	if step_size == 0.0 {
	log::trace!("Ignoring step_size of zero");

… though can we still get a panic when step_size is very small, or NaN? What was the actual panic? Can we fix it at the source instead of adding a guard like this?

[nrot]

Panic this extend. When step_size to small last - first = very big. And then allocated more memory than have system and this is panic in extend.