Phase plot in marginplot does not show dashed line at -180 degrees.

[lucasmsoares96]

Version: ControlSystems v1.5.1

using ControlSystems
using Plots
y2 = tf(
	[1],
	[1,13,40,0]
)
marginplot(y2,
	xticks=exp10.(-2:0.5:4),
)

the output of the code above is as follows:

In the magnitude graph, a dashed line at 0 is shown. In the phase graph, a dashed line at -180 degrees is not shown.

[baggepinnen]

Hello and thanks for the report! I think that the problem here comes from the fact that the system is believed to have infinite phase margin when considering only the frequencies over which the plot is drawn, the gain curve never crosses 1. If the gain of the system is increased, you get the phase guide at -180.

marginplot(10y2,
               xticks=exp10.(-2:0.5:4),
       )

There are thus two problems here

1. The phase margin is incorrectly computed due to the poorly chosen default frequency axis
2. No phase guide is shown unless there is a phase margin to draw

Providing a better frequency vector also resolves the problem

julia> w = exp10.(LinRange(-2, 2, 200));

julia> marginplot(y2, w,
               xticks=exp10.(-2:0.5:4),
       )