Matlab - Legend does not match my graph - matlab

Question

I am having trouble matching my graph with my axis. The first two plots work and the second two do not. I am trying to plot Temperature versus Pressure for two Argo floats and then Salinity versus Pressure. Here is my code:
% First Plot
subplot(221);
plot(float1winter.T,float1winter.P,'b');
hold on;
plot(float1summer.T,float1summer.P,'r');
hold on;
tempAdiff = abs(float1summer.T-float1winter.T)
plot(tempAdiff,float1summer.P,'--k');
hold on;
set(gca,'ydir','reverse');
title('Argo Float #1901440 Temp Profiles');
legend(['float1winter','float1summer','tempAdiff'],{'11-29-2013','07-01-2013','Temperature Difference'},'location','southwest');
xlabel('Temperature (°C)');
ylabel('Pressure');
shg;
% Second Plot
subplot(222);
plot(float2winter.S,float2winter.P,'m');
hold on;
plot(float2summer.S,float2summer.P,'c');
hold on;
set(gca,'ydir','reverse');
title('Argo Float #1901440 Salinity Profiles');
legend(['float2winter','float2summer'],{'11-29-2013','06-02-2013'},'location','southwest');
xlabel('Salinity (psu)');
ylabel('Presure');
shg;
% Third Plot
subplot(223);
% Matrix demensions did not agree bewteen winter and summer profiles. The summer profile was 71 x 2 and the winter was 70 x 2. I tried "reshape"
% and that didn't work. So I changed the vector of float3summer.T to
% float3bsummer.T with an array of 70 x 2
float3bsummer.T = float3summer.T(1:70,1:2);
float3bsummer.P = float3summer.P(1:70,1:2);
plot(float3winter.T,float3winter.P,'Linewidth',1,'color','blue');
hold on;
plot(float3bsummer.T,float3bsummer.P,'Linewidth',1,'color','red');
hold on;
tempdiff = abs(float3bsummer.T-float3winter.T)
plot(tempdiff,float3bsummer.P,'--k');
hold on;
set(gca,'ydir','reverse'); % this line reverses the y-axis so that depth increases downward
title('Argo Float #1901415 Tempearture Profiles');
hold on;
summerfloat = plot(float3bsummer.T,float3bsummer.P,'r');
legend(['float3winter.T','summerfloat','tempdiff'],{'12-03-2013','07-03-2013','Temp Diff'},'location','southeast');
xlabel('Temperature (°C)');
ylabel('Pressure');
axis ([-1,4,0,2000]);
shg;
% Fourth Plot
subplot(224);
plot(float3winter.S,float3winter.P,'g');
% Changed matrix dimensions for Salinity of Summer
float3bsummer.S = float3summer.S(1:70,1:2);
float3bsummer.P = float3summer.P(1:70,1:2);
plot(float3bsummer.S,float3bsummer.P,'.b');
hold on;
set(gca,'ydir','reverse');
title('Argo Float #1901415 Salinity Profiles');
h4 = plot(float3winter.S,float3winter.P,'g');
hold on;
h5 = plot(float3bsummer.S,float3bsummer.P,'.b');
hold on;
legend(['float3winter','float3bsummer.T'],{'12-03-2013','07-03-2013'},'location','southwest');
xlabel('Salinity (psu)');
ylabel('Pressure');
axis ([33.8,34.8,0,2000]);
shg;
% Save File to Desktop
set(gcf,'color','w');
saveas(gcf,'~/Desktop/hw1_figure1.pdf','pdf');![enter image description here][1]

Answer 1

I guess that you're trying to associate the set of strings for your legend, {'11-29-2013','07-01-2013','Temperature Difference'}, with the plots made from the variables ['float1winter','float1summer','tempAdiff'].
However, this isn't how legend works. MATLAB has no way of associating the plot produced by plot(float1winter.T,float1winter.P,'b'); to the string float1winter. If you want to specify which plots go with which legend entries, you need to pass the object handles of the plots to legend, which is easiest done by returning the handles when you plot originally:
h(1) = plot(float1winter.T,float1winter.P,'b');
hold on;
h(2) = plot(float1summer.T,float1summer.P,'r');
h(3) = plot(tempAdiff,float1summer.P,'--k');
legend(h,{'11-29-2013','07-01-2013','Temperature Difference'});
Side note: you only need to call hold on once per axis - so once for each subplot but not after every plot call.
Alternatively, you can not give handles at all; legend will assign the text to the plots in the order that they were plotted:
legend({'11-29-2013','07-01-2013','Temperature Difference'})
Understanding graphics handles allows you a lot more control over plots, especially if you might want to make small adjustments to them. For example, if I decide that I want the first plot to be green rather than blue, then I can just do:
set(h(1),'Color','g');
This will change the plot color and the legend changes to match automagically. To see a list of all the properties of an object, use get with only the handle. You can set more than one property at a time. For example:
get(h(1))
set(h(1),'DisplayName','Winter','LineWidth',3,'Marker','x')