How to edit the "▲" marks on the Nyquist plot and change only the lines in the negative frequency range to dashed lines

Is there a way to change the size and color of the ▲ marks that appear when drawing a Nyquist diagram?
Is there a way to change thae style of line of the line in the negative frequency when drawing a Nyquist diagram?

Answer 1

[Walter Roberson]

nqp = findobj(groot, 'type', 'nyquist');

npa = findobj(nqp, 'Tag', 'NyquistPositiveArrow');
npa.FaceColor = APPROPRIATE_RGB_TRIPLE;
npa.EdgeColor = APPROPRIATE_RGB_TRIPLE;

nna = findobj(nqp, 'Tag', 'NyquistNegativeArrow');
nna.FaceColor = APPROPRIATE_RGB_TRIPLE;
nna.EdgeColor = APPROPRIATE_RGB_TRIPLE;

To change the size, you have to change npa.Vertices and nna.Vertices to reflect new data-relative coordinates. Something like

npa_centroid = mean(npa.Vertices, 1);
npa.Vertices = npa_centroid + (npa.Vertices - npa_centroid) * SCALE_FACTOR;

[ayumi obitsu]

Thanks for your answer.

I get an error saying nqa is empty.

Why is that?

Also, is there a way to make the Nyquist plot at negative frequencies a dashed line?

[Walter Roberson]

This code assumes that you currently have a nyquist() plot showing.

With respect to plotting with dashed lines:

nrl = findobj(nqp, 'Tag', 'NyquistResponseLine');

That is the handle of a single line() object with XData and YData properties and LineStyle property.

In order to get a dashed line, you would need to locate the part of XData where it goes negative again, and plot sections of XData and YData. That is, if there is any negative X at all, XData starts out negative and goes more positive, but eventually starts decreasing until it becomes negative again, after which it decreases. You would need to plot() the leading negative XData and corresponding YData with dashed line, then plot the positive XData and corresponding YData with solid line, then plot the trailing negative XData and corresponding YData with dashed line -- three lines. You would need to specify the parent to plot into, which would be nrl.Axes; you would probably need to hold(nrl.Axes,'on') .

Or instead of plotting three lines, you could extract the trailing negative XData and corresponding YData, and append to that the leading negative XData and corresponding YData, and plot that as a single dashed line.

It is worth thinking about what the appropriate Tag would be for those lines. And about whether the datatips will continue to function afterwards.

[Paul]

The XData and YData are the real and imaginary parts of a complex function of frequency H(w). The OP wants the portion of H(w) corresponding to w < 0 to be dashed.

I don't know if the frequency vector, w, is stored inside the graphics object created from either nyquist or nyquistplot. It probably is for the latter to support functionality of the ShowFullContour property.

Conceptually, one way to do this would be to make one plot for w>=0 and then add another plot into the same graphics object for w < 0 with the desired line type. Whether or not that will actually work is a different question. CST graphics object are not easy to work with.

Perhaps something like this to start, which overlays the same Nyquist plot twice. Then maybe the XData and YData of each could me manipulated to delete the w < 0 points from the first and the w >= 0 points from the second, with appropriate operations on the triangles as well. No idea what havoc that might wreak on any other functionality. Perhaps something similar might be feasible with nyquistplot.

H = tf(1,[1 2 3 4]);
w = logspace(-2,3,5000);
figure
nyquist(H,w);
hold on
nyquist(H,w,'r--')

Probably the easiest way to do this would be to use the output argument form of nyquist and then make the plot from the outputs w/ whatever styling is desired.

[ayumi obitsu]

Thanks for the reply.

The XData and YData are the real and imaginary parts of a complex function of frequency H(w). The OP wants the portion of H(w) corresponding to w < 0 to be dashed.

>>That is correct.

I found that it is not easy to edit the size and color of ▲, or to make the Nyquist diagram of the negative area frequencies dashed, both of which are not easy to do.

I will keep trying.

If you figure out how to do it, I would appreciate it if you could let me know.

[Walter Roberson]

Setting color:

npa = findobj(groot, 'Tag', 'NyquistPositiveArrow');
npa.FaceColor = APPROPRIATE_RGB_TRIPLE;
npa.EdgeColor = APPROPRIATE_RGB_TRIPLE;

Setting size:

The internal representation of nyquist() plots does not draw the arrows as markers of any kind. The internal representation of nyquist() plots draws the arrows as patch() objects. I already posted code that should rescale the patch objects.

Answer 2

[Paul]

"Probably the easiest way to do this would be to use the output argument form of nyquist and then make the plot from the outputs w/ whatever styling is desired."

This doesn't give you all the bells and whistles of nyquist or nyquistplot, but it's a start. It uses @Walter Roberson's sleuthing to get the arrows.

I assumed that the 'PositiveArrow' applied for positive frequencies, and the 'NegativeArrow' for negative frequencies, but apparently that's not the case. I'll leave it to you to make the arrows however you want.

h = tf(1,[1 2 3 4]);
[hreal,himag,w] = nyquist(h);
hreal = squeeze(hreal);himag = squeeze(himag);
figure
hax = gca;
plot(hax,hreal,himag,'b-',hreal,-himag,'r--'),grid
hold on
plot(hax,-1,0,'r+','MarkerSize',15)
xlim('padded')
hf = figure('Visible','off');
hny = nyquistplot(gca,h);
harrow = copyobj([findobj(hny, 'Tag', 'NyquistPositiveArrow'),findobj(hny, 'Tag', 'NyquistNegativeArrow')],hax);
harrow(1).FaceColor = 'b';harrow(1).EdgeColor = 'b';
harrow(2).FaceColor = 'r';harrow(2).EdgeColor = 'r';

delete(hf);clear hf hny