How do I changethe label in a compass graph

Hello

Here is my code , I find it in the documentation.

I would change the label like here : https://fr.mathworks.com/help/matlab/creating_plots/compass-labels-on-polar-axes.html

but I always get a mathlab error and I don't understand why.

Thank's , Olivier

    % Channel ID to read data from 
readChannelID = 926212; 
WindDirFieldID = 4;
WindSpeedFieldID = 3;
readAPIKey = 'XXXXXXXXXXXXXXX'; 

windDir = thingSpeakRead(readChannelID,'Fields',WindDirFieldID,'NumPoints',10,...
'ReadKey',readAPIKey);

windSpeed = thingSpeakRead(readChannelID,'Fields',WindSpeedFieldID,'NumPoints',10,...
'ReadKey',readAPIKey);

rad = windDir*2*pi/360;
rad = (rad+pi/2);

u = -cos(rad) .* windSpeed; 
v = sin(rad) .* windSpeed; 
title('anemo');

%pax = gca;
angles = 0:45:360;
pax.ThetaTick = angles;
labels = {'E','NE','N','NW','W','SW','S','SE'};
pax.ThetaTickLabel = labels;
compass(u,v);

Answer 1

[dpb]

Well, the above code commented out

pax=gca;

so the handle to the axes is either undefined or refers to a probably no longer existing object if the variable exists from a previous run. Or, if there is no variable pax the first time the code is run it will be created by the dot notation as a struct:

windSpeed=randi(30,size(windDir));
windDir=deg2rad(randi(360,10,1)-1);
u = -cos(rad) .* windSpeed; 
v = sin(rad) .* windSpeed; 
title('anemo');
%pax = gca;
angles = 0:45:360;
pax.ThetaTick = angles;
labels = {'E','NE','N','NW','W','SW','S','SE'};
pax.ThetaTickLabel = labels;
compass(u,v);

executed when there was no variable pax in the workspace resulted in

>> pax
pax = 
  struct with fields:

         ThetaTick: [0 45 90 135 180 225 270 315 360]
    ThetaTickLabel: {'E'  'NE'  'N'  'NW'  'W'  'SW'  'S'  'SE'}
>> 

But, the bigger problem is that the example is for polarplot which creates a PolarAxes object and you drew a compass plot which uses a regular Cartesian axes object which does not have the properties 'ThetaTick', 'ThetaTickLabel' but computes and draws a polar coordinate system axis/lines in cartesian coordinates.

A quick perusal didn't uncover just how those are done or where those properties might be buried but they're not easily accessible it seems.

Alternatively, you might try to draw a polarplot() object to label and then add the compass plot on top...but I'm not sure how you'd turn off the labels...and I don't have time to delve more deeply at the moment, sorry.

This looks like a Q? to official support, mayhaps with a request for enhancement to get compass() to also use the polar axes object.

[olivier task]

thank you for the answer.

I'm not a specialist so I will left it as it is. For the moment it's enough for my project.

Best regards. Olivier

[dpb]

Well, I did find where you can get access to stuff, but it's a bugger to do anything with.

compass is a builtin function so you can't get access to the internal code and it is completely devoid of visible user interaction of the arrangement of the polar coordinate graph -- the arbitrary 30 degree angles for the rays is hardcoded.

It does look like with a whole lot of work one could manage to do the things wanted by physically deleting the 30,60 degree lines and drawing another at the 45 for each of the quadrants, but again have to do that by retrieving the internal range values and computing those positions in polar coordinates from the cartesian values.

There are some 25 other line and text objects besides the lines themselves that make up the coordinate display; all are under the

hAx=gca;
hNodeChildren=hAx.ActiveChildContainer.NodeChildren;

returns an array of handles to all the underlying pieces of the graph.

[olivier task]

Thank you for your investment. I am finally using polarplot. I can't draw the arrow, but the plot is enough.

https://thingspeak.com/apps/matlab_visualizations/319108?size=iframe

Best regards. Olivier

[dpb]

Don't blame you... :)

You could probably add annotations for the arrowheads to polarplot although I've not explored that option.