How to add different colours to positive and negative regions of area plot?

Hi, I am trying to make an area plot with separate colours for positive and negative regions. I tried separating the positive and the negative parts and then plotted both one upon another:

for i=1:length(mismatch)
  if (mismatch(i) < 0)
      neg_mismatch(i)=mismatch(i);
      pos_mismatch(i)=0.0;
  else
      neg_mismatch(i)=0.0;
      pos_mismatch(i)=mismatch(i);
  end
end

The problem is there are overlap regions of positive and negative values around 0 values (see attachment) which makes the plot look ugly. I tried to interpolate values in between, but still no improvement!

     x_neg=(1:length(neg_mismatch))';
     y_neg=neg_mismatch;
     xi_neg=(1:0.01:length(neg_mismatch))';
     yi_neg = interp1(x_neg,y_neg,xi_neg,'cubic');
     x_pos=(1:length(pos_mismatch))';
     y_pos=pos_mismatch;

     xi_pos=(1:0.01:length(pos_mismatch))';
     yi_pos = interp1(x_pos,y_pos,xi_pos,'cubic');

     figure;

     hold on
     h1=area(yi_neg);
     h2=area(yi_pos);
     hold off

Can anyone please help!

Answer 1

[WAT]

The problem is that the area command is drawing lines from the actual data points to the zeroed out data, rather than connecting to the actual (but opposite signed) data point.

As a simple example, think of what would happen if you tried to do a positive/negative area plot for the simple two-point data set [-1,1].

There may be a more elegant solution, but one way is to insert a fake 0 data point wherever the data switch from positive to negative (or vice-versa):

 %%MAKE UP SOME DATA
 x = -10:.1:10;
 y = sinc(x)+0.2*random('normal',0,1,1,length(x));
 plot(y)

 %%TRY SPLITTING IT THE OLD WAY
 y1=y;y2=y;
 y1(y1<0)=0; % zero out negative entries
 y2(y2>0)=0; % zero out positive entries
 figure
 hold on
 h1 = area(x,y1,'FaceColor','b')
 h2 = area(x,y2,'FaceColor','r');

 %%NOW INSERT ZEROS WHERE LINE CROSSES THE Y AXIS

 % Insert 0s wherever data transitions from +/- or -/+
 count = 0; % counter for number of fake 0's inserted
 for ii=1:length(y)-1

   i = ii+count; % i is the index in our new, possibly extended vectors

   % find points with different signs
   if(y(i+1)*y(i) < 0) 
       % find y intercept
       x_fake = x(i) + abs(y(i))/(abs(y(i))+abs(y(i+1))) * (x(i+1)-x(i)); 

       % put fake data into x and y arrays
       x = [ x(1:i), x_fake , x(i+1:length(x))];
       y = [ y(1:i), 0 , y(i+1:length(y))];

       % increment counter
       count = count+1;

   end  
end

 % create y1 and y2, consisting of nonnegative and nonpositive entries
 % of y respectively
 y1=y;  y2=y;
 y1(y1<0) = 0;
 y2(y2>0) = 0;

 figure
 hold on
 area(x,y1,'FaceColor','b');
 area(x,y2,'FaceColor','r');
 hold off

[Kabitri Chattopadhyay]

Thanks a lot for nice explanation and your code. It works perfectly!!!