only the first of the three subplots shows eveything including the points, grid, axis labels, and legend. The last two subplots only show the trend line and none of the extra features

subplot(3,1,1); plot(mass,quarter,'rx'), grid on, xlabel('Load/g'), ylabel('Voltage/v'),

hold on, plot(mass,half,'bx'), plot(mass,full,'gx'), legend('Quarter Bridge','Half Bridge','Full Bridge'),

line_quarter=polyfit(mass,quarter,1); plot_quarter_trend=polyval(line_quarter,mass); plot(mass,plot_quarter_trend,'r-'),

line_half=polyfit(mass,half,1); plot_half_trend=polyval(line_half,mass); plot(mass,plot_half_trend,'b-'),

line_full=polyfit(mass,full,1); plot_full_trend=polyval(line_full,mass); plot(mass,plot_full_trend,'g-'),

subplot(3,1,2); plot(mass,amp,'mx'), grid on, xlabel('Load/g'), ylabel('Voltage/v'), legend('Amplifier'),

line_amp=polyfit(mass,amp,1); plot_amp_trend=polyval(line_amp,mass); plot(mass,plot_amp_trend,'m-'),

subplot(3,1,3); plot(mass,div,'cx'), grid on, xlabel('Load/g'), ylabel('Voltage/v'), legend('Potential Divider'),

line_div=polyfit(mass,div,1); plot_div_trend=polyval(line_div,mass); plot(mass,plot_div_trend,'c-')

Answer 1

[Aashray]

Hi Jacob Cole,

In the first subplot, you explicitly did a hold on, so MATLAB kept all the plotted objects (quarter, half, full, and their trend lines). That’s why all markers + lines showed up in the legend.

But in the 2nd and 3rd subplots, there was no hold on.

So, what happened is after plotting the 'mx' markers, the trend line (‘m-‘) line was immediately plotted. And since hold on wasn’t active, the second plot command replaced the first one on that subplot. That’s why the markers and legend entries for “Amplifier” or “Divider” weren’t visible.

To solve this, you can add hold on before plotting the trend line and use DisplayName with each plot instead of hardcoding legend early. Then call legend('show') at the end as shown below:

% Dummy data (since your data wasn’t available, I am generating some synthetic data on my own) 
mass = (0:50:500)'; 

% Create synthetic linear-ish responses with some noise 
quarter = 0.002*mass + 0.05*randn(size(mass)); 
half    = 0.003*mass + 0.05*randn(size(mass)); 
full    = 0.004*mass + 0.05*randn(size(mass)); 
amp     = 0.010*mass + 0.1*randn(size(mass)); 
div     = 0.0015*mass + 0.05*randn(size(mass)); 

subplot(3,1,1); 
plot(mass,quarter,'rx'), grid on, xlabel('Load/g'), ylabel('Voltage/v'), 
hold on, plot(mass,half,'bx'), plot(mass,full,'gx'), 
legend('Quarter Bridge','Half Bridge','Full Bridge'), 
line_quarter=polyfit(mass,quarter,1); 
plot_quarter_trend=polyval(line_quarter,mass); 
plot(mass,plot_quarter_trend,'r-'), 
line_half=polyfit(mass,half,1); 
plot_half_trend=polyval(line_half,mass); 
plot(mass,plot_half_trend,'b-'), 
line_full=polyfit(mass,full,1); 
plot_full_trend=polyval(line_full,mass); 
plot(mass,plot_full_trend,'g-'), 
hold off 

subplot(3,1,2); 
plot(mass,amp,'mx'), grid on, xlabel('Load/g'), ylabel('Voltage/v'), 
legend('Amplifier'), 
hold on 
line_amp=polyfit(mass,amp,1); 
plot_amp_trend=polyval(line_amp,mass); 
plot(mass,plot_amp_trend,'m-'), 
hold off 

subplot(3,1,3); 
plot(mass,div,'cx'), grid on, xlabel('Load/g'), ylabel('Voltage/v'), 
legend('Potential Divider'), 
hold on 
line_div=polyfit(mass,div,1); 
plot_div_trend=polyval(line_div,mass); 
plot(mass,plot_div_trend,'c-'), 
hold off

●

You can observe that the legends and markers are visible in 2nd and 3rd plots in the above figure.