Why am I Unable to use Datetime Ticks for UIAxes in MATLAB App Designer?

I am working on creating a tool that collects data and plots the data vs. time in real time. I am trying to plot the x-axis as a datetime value. The data is formatted as a datetime array. However, when I pass the data as part of the plot() function, I get an error saying the data must match the numeric format of the x-axis. I did some searching online and saw suggestions using the datetick() or xtickformat() functions to format x-axis as a datetime. I am still getting an error: Error using xtickformat (line 34)Invalid numeric tick label format.Error in Wifi_Performance_Tool (line 643) runStartupFcn(app, @startupFcn). Is it possible to format the x-axis as a datetime.

Here is my startupfcn code:

try
    app.loadState;
catch                
end

xtickformat(app.UIAxes,'MMM d, yyyy HH:mm:ss a');
while isvalid(app.UIFigure)
    if strcmp(app.Switch.Value,'On')
        status = readData(app);
        if status
        	writeAPFields(app);
        	writeCurrentConditions(app);
        	moveGauge(app);
        	plotData(app);
        	drawnow;
        	pause(0.5);
        else
            plotData(app);
        	drawnow;
            pause(1.0);
        end
    else
        pause(1.0);
    end
end

Here is my UIAxes properties:

% Create UIAxes
app.UIAxes = uiaxes(app.UIFigure);
xlabel(app.UIAxes, 'Time')
app.UIAxes.PlotBoxAspectRatio = [2.63428571428571 1 1];
app.UIAxes.FontWeight = 'bold';
app.UIAxes.XTick = [];
app.UIAxes.BoxStyle = 'full';
app.UIAxes.FontSize = 16;
app.UIAxes.Position = [39 19 1289 521];

Here is the code I am using to plot one line:

hold(app.UIAxes,'on');
if app.RSSICheckBox.Value
    plot(app.UIAxes,datenum(app.Datetime_all),app.RSSI_all,'-c',"Marker","o","MarkerFaceColor","cyan","Visible","on")
else
    plot(app.UIAxes,datenum(app.Datetime_all),app.RSSI_all,'-c',"Marker","o","MarkerFaceColor","cyan","Visible","off")
end
hold(app.UIAxes,'off');
drawnow;

Also, if anyone can hep, I have check boxes in the toll that should make each line visible or hidden depending on its value. However, the plot becomes visible when I select the checkbox, but when I deselect the box the plot does not become hidden, it stays visible. Any help on these issues would be much appreciated

[dpb]

 plot(app.UIAxes,datenum(app.Datetime_all),app.RSSI_all,'-c',"Marker","o","MarkerFaceColor", ...

will create a NumericRuler x-axis, not a DatetimeRuler. Hence, if you then try to plot a datetime value you'll get the error it must be numeric.

I've got to run right now so can't test, in earlier releases, once you created a numeric axis it was very difficult to turn it into a datetime one; you needed to make the very first plot/creation with a datetime value, even if it was a dummy to create the axes object.

I do not know if it's easier with recent version, but ensure that if you use datetime, use it EVERYWHERE including the first and every time you try to plot.

Answer 1

[Walter Roberson]

It is possible to plot datetime in app designer.

dt = datetime('now') - days(1:5)
r = rand(1,5)
fig = uifigure()
ax = uiaxes(fig)
plot(ax, dt, r)

What is not possible is to plot datetime data on an axes that already has non-datetime plotted on it.

hold(app.UIAxes,'on');

You should debug at that point to see if app.UIAxes already has plotted information.

[Nick Nauman]

I put a breakpoint in the startup function right after the app is loaded and before any plotting happens. I accessed the properties in the command window and the XAxis Property is already set as a Numeric Ruler which is why I am unable plot any datetime values on the x-axis. Everything I have seen online, suggests using a function like datetick() or xtickformat() to set the axis to a datetime ruler, but I get an error when I try to call these functions because the x-axis is a numeric ruler. Is there any way to change this to a Datetime Ruler? I added the Axes using Design View when I built the app format and I do not see anything in the configuration options to do so. Also, note, all the data I am trying to plot is plotted as datetime vs. data. I have included the code that shows the datenum(Datetime_all) because I was testing other features of the plot at the time. I still get the error when all x data of plot() function are Datetime_all as well.

[Walter Roberson]

It is normal for a newly created axes to be given NumericRuler children. You can draw datetime plots in the axes if:

• the NextPlot property of the axes is 'replace' (this is equivalent to "hold" being "off"); or
• the appropriate ruler is already a DateTime Ruler; or
• there are no children of the axes

Have a look in your printout at XTick being emtpy. That is not the default . Something has to have changed the xticks.

[Nick Nauman]

Thank you very much for all the help; I appreciate it a lot (i'm not very familiar with matlab app designer yet)! I added a line before my while loop to plot a datetime random series like the code you presented, but I made the line hidden. This allowed me to plot all other datetime lines without any error. I do have one more question that I'm hoping you can answer that isn't exactly related to the datetime series. My gui has checkboxes that are meant ot control if the line is visible or hidden on the uiaxes plot. When the app starts, all lines are hidden and when I select a checkbox the line appears in the uiaxes. However, when I deselect the checkbox, the line does not become hidden. This is the plot code I am using (same code for all 7 lines)

%app.SNRPlot = plot(app.UIAxes,app.Datetime_all,app.SNR_all,'-m',"Marker","o","MarkerFaceColor","magenta");
if app.SNRCheckBox.Value
    hold(app.UIAxes,'on');
    plot(app.UIAxes,app.Datetime_all,app.SNR_all,'-m',"Marker","o","MarkerFaceColor","magenta","Visible","on")
    hold(app.UIAxes,'off');
else
    hold(app.UIAxes,'on');
    plot(app.UIAxes,app.Datetime_all,app.SNR_all,'-m',"Marker","o","MarkerFaceColor","magenta","Visible","off")
    hold(app.UIAxes,'off');
end
drawnow;

I have found the issue to be the hold function keeps me from removing the visible line and replacing it with the hidden line. If I remove the hold function from around the plot function with "Visible","off" property then when I am running the plot in realtime, the line gets redrawn everytime causing the x-axis to resize to what it was at app startup. This causes a flickering that is not visually appealing. I tried every next plot property (since I am not familiar with what each one does), but the visible line does not change to hidden. I also did not have luck with assigning a line object and changing the visibility property everytime the checkbox value changes. Is anyone aware of a good solution to this problem? Any advice is much appreciated.

[Walter Roberson]

You should keep a copy of the handles of the line() objects returned from plot(), and then you should set the handle Visible property on or off as needed, without redrawing it. Only redraw it when the data changes (at most)... better yet if the data changes, set the XData or YData properties to update the data without using plot() again.