How can I plot a legend only for data points?

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Rytis Stockus 2018-10-8

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https://ww2.mathworks.cn/matlabcentral/answers/422804-how-can-i-plot-a-legend-only-for-data-points

评论： jonas 2018-10-9

采纳的回答： jonas

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Hello I try to plot a legend for different data points and get this error:

Unable to perform assignment because the left and right sides have a different number of elements.

Error in Robertson86 (line 57) h(i)=plot(xdata(in),ydata(in),'o','Color',cdata2(i,:)); But it does work with line plots and fills.

for i = 1:12

    % plot a polygon
    Poly = Robertson_zones_1986{i};
    plot(Poly(:,1),Poly(:,2),'k','LineWidth',1.5)
    fill(Poly(:,1),Poly(:,2),cdata(i,:))
    text(RobNumPos86(i,1),RobNumPos86(i,2),num2str(i),'Color',[0.3 0.3 0.3])
    % check if data is inside this particular polygon
    in= inpolygon(xdata,ydata,Poly(:,1),Poly(:,2));
    h(i)=plot(xdata(in),ydata(in),'o','Color',cdata2(i,:));
    b(i)=numel(xdata(in));
end
legend(h,'1.Data type','2.Data type',....)

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Answer 1

jonas 2018-10-8

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Your plot outputs multiple line handles. If that is not what you want, then you need to adjust the dimensions of xdata and ydata. If you want to plot multiple line objects in every iteration, then I suggest you store those values in a cell array:

h{i}=plot(xdata(in),ydata(in),'o','Color',cdata2(i,:));

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jonas 2018-10-9

编辑：jonas 2018-10-9

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The problem is that you have no points inside of some of the polygons. Therefore, no line handle is produced and the script returns an error when it tries to assign an empty array to h(i). Try something like this

figure;hold on
for i = 1:12
    % plot a polygon
    Poly = Robertson_zones_1986{i};
    plot(Poly(:,1),Poly(:,2),'k','LineWidth',1.5)
    m=fill(Poly(:,1),Poly(:,2),cdata(i,:))
    text(RobNumPos86(i,1),RobNumPos86(i,2),num2str(i),'Color',[0.3 0.3 0.3])
      % check if data is inside this particular polygon
      in= inpolygon(xdata,ydata,Poly(:,1),Poly(:,2));
      if sum(in)>0;
          h(i)=scatter3(xdata(in),ydata(in),ones(size(ydata(in))),[],cdata(i,:),'filled','markeredgecolor','r','DisplayName',sprintf('Data type %g',i));
          b(i)=numel(xdata(in));
      else
          h(i)=NaN
      end
end
legend(h(~isnan(h)))

I dont know how you want to deal with the fact that some data types are absent. Do you want to indicate this in the legend somehow? Also, note that I changed the 'cdata2(i,:)' that was inside of the plot to 'cdata(i,:)' as I did not find a variable called cdata2.

jonas 2018-10-9

编辑：jonas 2018-10-9

在 MATLAB Online 中打开

Exactly. An interesting observation I made was that neither of

plot([],[])
scatter([],[])

returns a handle. Therefore, if you try to assign them to h(i), they will both return an error. However, if you write like this

plot([],[],'color','r')
scatter([],[],'markerfacecolor','r')

then the second one (scatter) actually returns a handle. I don't know why this is the case. But if you want to show all datatypes, even those not present in the plot, then you can simply remove the if-condition, such as

figure;hold on
for i = 1:12
  % plot a polygon
  Poly = Robertson_zones_1986{i};
  plot(Poly(:,1),Poly(:,2),'k','LineWidth',1.5)
  m=fill(Poly(:,1),Poly(:,2),cdata(i,:))
  text(RobNumPos86(i,1),RobNumPos86(i,2),num2str(i),'Color',[0.3 0.3 0.3])
  % check if data is inside this particular polygon
  in= inpolygon(xdata,ydata,Poly(:,1),Poly(:,2));  
  h(i)=scatter3(xdata(in),ydata(in),ones(size(ydata(in))),[],cdata(i,:),'filled','markeredgecolor','r','DisplayName',sprintf('Data type %g',i));
  b(i)=numel(xdata(in));
end
legend(h)

Rytis Stockus 2018-10-9

Yes, your solution does work as I wished. Thanks again for your help!

jonas 2018-10-9

Cheers!

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How can I plot a legend only for data points?

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How can I plot a legend only for data points?

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