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I've got a 2-D histogram (the plot is 3D - several histograms graphed side by side) that I've generated with the bar3 plot command. However, all the zero values show up as flat squares in the x-y plane. Is there a way I can prevent MATLAB from displaying the values? I already tried replacing all zeros with NaNs, but it didn't change anything about the plot. Here's the code I've been experimenting with:

x1=normrnd(50,15,100,1); %generate random data to test code
x2=normrnd(40,13,100,1);
x3=normrnd(65,12,100,1);

low=min([x1;x2;x3]);
high=max([x1;x2;x3]);
y=linspace(low,high,(high-low)/4); %establish consistent bins for histogram
z1=hist(x1,y);
z2=hist(x2,y);
z3=hist(x3,y);
z=[z1;z2;z3]';
bar3(z)

As you can see, there are quite a few zero values on the plot. Closing the figure and re-plotting after replacing zeros with NaNs seems to change nothing:

close
z(z==0)=NaN;
bar3(z)
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2 Answers

up vote 6 down vote accepted

One solution is to modify the graphics objects created by BAR3. First, you have to get the handles returned from BAR3:

h = bar3(z);

In your case, h will be a 3-element vector of handles, one for each set of colored bars. The following code should then make the bins with counts of zero invisible:

for i = 1:numel(h)
  index = logical(kron(z(:,i) == 0,ones(6,1)));
  zData = get(h(i),'ZData');
  zData(index,:) = nan;
  set(h(i),'ZData',zData);
end

And here's an illustration (with obligatory free-hand circles):

alt text

How it works...

If your vector of bin counts is N-by-1, then BAR3 will plot 6*N rectangular patches (i.e. the 6 faces of a cuboid for each bin). The 'ZData' property for each set of patch objects in h will therefore be (6*N)-by-4, since there are 4 corners for each rectangular face. Each cluster of 6 rows of the 'ZData' property is therefore a set of z-coordinates for the 6 faces of one bin.

The above code first creates a logical vector with ones everywhere the bin count equals 0, then replicates each element of this vector 6 times using the KRON function. This becomes an index for the rows of the 'ZData' property, and this index is used to set the z-coordinates to NaN for the patches of empty bins. This will cause the patches to not be rendered.

EDIT:

Here's a slightly modified version of the code that makes it more general by fetching the bar height from the 'ZData' property of the plotted bars, so all that's needed for it to work are the handles returned from BAR3. I've also wrapped the code in a function (sans error and input checking):

function remove_empty_bars(hBars)
  for iSeries = 1:numel(hBars)
    zData = get(hBars(iSeries),'ZData');  %# Get the z data
    index = logical(kron(zData(2:6:end,2) == 0,ones(6,1)));  %# Find empty bars
    zData(index,:) = nan;                 %# Set the z data for empty bars to nan
    set(hBars(iSeries),'ZData',zData);    %# Update the graphics objects
  end
end
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I got the idea to modify the object properties after I asked the question, but I didn't know where to start. Your code works great - I put it in an m-file bar3nonzero, and now I've got a hassle free way to produce plots. – Doresoom Jan 12 '10 at 18:03
@gnovice: +1 ..yet again you managed to find a good use for the kron function :) – Amro Jan 12 '10 at 22:51
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up vote 1 down vote

My problem was not zero values, but NaN values (which are converted into zero values inside of bar3). I wanted to keep displaying elements with values zero, but not the elements with value nan. I adjusted the code slightly, and it worked perfectly:

for i = 1:numel(h)
  index = logical(kron(isnan(z(:,i)),ones(6,1)));
  zData = get(h(i),'ZData');
  zData(index,:) = nan;
  set(h(i),'ZData',zData);
end

Thanks!

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