# Is there a way to put a marker on all of the points where y=0 on a plot?

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Maddie Sanders on 16 Feb 2020
Commented: Sindar on 16 Feb 2020
I have a plot and would like to highlight/mark the points where y=0. Is there anyway to do this without knowing the x-value? Also is there anyway to label these points with the x and y value on the plot?

Sindar on 16 Feb 2020
Do you have the data used to create the plot?
Walter Roberson on 16 Feb 2020
Note that the above will only work for y values that are exactly 0. Depending on your needs, you might want to test for "near" zero, such as
idx = abs(y) < 1e-6;
But it is also common to have a vector of x and y values where lines joined between them cross y = 0 somewhere rather than very close to an associated x value. In such cases, you may have to interpolate to figure out where the zero is (if all you have is data) or you may need to fzero() to find the zero (if you have a function).

Sindar on 16 Feb 2020
Assuming you have access to the data (x,y) which generated the plot:
% generate sample x,y
x=1:1000;
% generate 1000 random integers between -5 and 5
y=randi([-5,5],1000)
% plot the data as a black line
plot(x,y,'k')
% find indices where y is 0
idx = (y==0);
% add to the existing plot, with red asterisks where y is zero
hold on
myplot = plot(x(idx),y(idx),'*r')
% with Matlab 2019b, you can programmatically label these points
for ind=idx
datatip(myplot,x(ind),y(ind))
end

#### 1 Comment

Sindar on 16 Feb 2020
you may want to add a tolerance if your data is not generated in a way where it will be exactly zero:
idx = ( abs(y) < 1e-10 );