Why can't I display a line on a plot (can only have points marker)

I'm trying to plot a line (x1,y1) on a plot where I applied a mask for land (green) and seas (blue). Whatever the LineSpec I try to use it only displays marker points instead of a line.

I tried to plot the three different options below (plt1, plt2, plt3). I get (x1,y1) in the first case, but it displays points instead of a continuous line as expected (matlab_fig1.png). In the second (matlab_fig2.png) and third case (matlab_fig3.png), I don't get anything.

Here is my code:

%Plot in cartesian coordinates

%Make Land mask
load([pwd '/Grid/bathyEASE.mat']); % load bathymetry
Land=NaN(361,361);
Land(bathyEASE>0)=1;
Land(bathyEASE<0)=0;

%plot
nexttile
hold on; box on;
xlim([0 361]);
ylim([0 361]);
p=pcolor(Land);
colormap(winter);
set(p,'facealpha',0.5)
shading flat;
axis ij;

x1 = [159:163];
y1 = 77;

plt1 = plot(x1,y1,'-r','LineWidth',3,'Marker','.','MarkerEdgeColor','r');  %Refer to --> matlab_fig.png
%plt2 = plot(x1,y1,'-r','LineWidth',3);                                    %Refer to --> matlab_fig2.png
%plt3 = plot(x1,y1);                                                       %Refer to --> matlab_fig3.png

Thank you

Answer 1

[dpb]

x1 = [159:163];
y1 = 77;
plt1 = plot(x1,y1,'-r','LineWidth',3,'Marker','.','MarkerEdgeColor','r');

When you plot() a vector of one variable against a constant in the other, plot creates a line object for each element of the vector and a single pixel marker simply isn't large enough to show up; there is no line created because there are (in this case 5) separate line objects, each consisting of only a single point.

You also have the sequence of commands in wrong order; or you need a hold on first after setting the axes limits and orientations or plot will cause refreshing the limits of the plot to match the plotted data.

If you want to plot a single vertical or horizontal line; the xline or yline functions are the better way to do so; they're relatively recent but get around the need to duplicate the constant data value manually (but cross the entire axis in extent so not helpful here -- Addendum/Correction --dpb)

Try something more like

xlim([0 361]),ylim([0 361])
axis ij
x=[159:163];
y=77;
hL=plot(x,repmat(y,size(x)),'-r','LineWidth',3); % ammended remove yline call -- dpb

and joy should ensue.

[A LL]

Thanks dpb for your help!

I tried the yline function as you mentionned but I got the following error:

Error using yline (line 29)

Passing multiple values to ConstantLine is not supported.

Error in define_domain (line 50)

hL1=yline(x1,y1,'-r','LineWidth',3.5);

Since, yline seems to only accept a single y value as an argument I wrote:

x1 = [159:163];
y1 = 77;
hL1=yline(y1,'-r','LineWidth',3.5);

I do get a constant line as expected but now I can't seem to find how to impose xlim to yline and not have a constant line for all my domain (see attached figure: matlab_fig4.png).

Do you know how I can impose a limit in y while using yline?

Thanks

[dpb]

Oh, yeah, I forget (that's what happens with age, it seems, I don't recommend it! :) )

Indeed, x/yline do go from one edge of the axes to the other -- but note that you only need the first and last points to draw a straight line, not all the intermediary ones.

xlim([0 361]),ylim([0 361])
axis ij
hold on
x=[159,163];
y=repmat(77,size(x));
hL=plot(x,y,'-r','LineWidth',3);

[A LL]

Right, this is clever! Thank you dpb :)