How can I divide the y-axis of a plot into two different color regions?

I am trying to divide a plot into green and red regions orientated horizallay across the y-axis and at a specific point on an eponential curve. For example, I would like the graph background to be green below a value of 5 on the y-axis of a curve and red above that value. Here is what I have tried so far,

clc

clc

clear

x=1:10;

y=[1 3 5 7 7.2 7.3 7.5 8 8.5 9];

plot(y,x)

%cftool(y,x)

x=0:0.01:10;

a=0.5552;

b=0.3237;

f_x=a*exp(b*x);

plot(x,f_x)

low=0:5; %I would like the plot background to be green in this range

high=5:10; %I would like the plot background to be red in this range

ciplot(low,high,'b');

%function ciplot(lower,upper,x,colour);

% ciplot(lower,upper)

% ciplot(lower,upper,x)

% ciplot(lower,upper,x,colour) % % Plots a shaded region on a graph between specified lower and upper confidence intervals (L and U).

% l and u must be vectors of the same length.

% Uses the 'fill' function, not 'area'. Therefore multiple shaded plots

% can be overlayed without a problem. Make them transparent for total visibility.

% x data can be specified, otherwise plots against index values.

% colour can be specified (eg 'k'). Defaults to blue.

% Raymond Reynolds 24/11/06

%if length(lower)~=length(upper) %error('lower and upper vectors must be same length')

%end

%if nargin<4

    %colour='b';

%end

%if nargin<3

 %   x=1:length(lower);

%end

% convert to row vectors so fliplr can work

%if find(size(x)==(max(size(x))))<2

%x=x'; end

%if find(size(lower)==(max(size(lower))))<2

%lower=lower'; end

%if find(size(upper)==(max(size(upper))))<2

%upper=upper'; end

%fill([x fliplr(x)],[upper fliplr(lower)],colour)

%--Error Message--%

%Error using fill

%String argument is an unknown option.

%Error in ciplot (line 36)

%fill([x fliplr(x)],[upper fliplr(lower)],colour)

%Error in example_yaxis_divide (line 21)

%ciplot(low,high,'b');

Answer 1

[Geoff Hayes]

You could try to use the area command to fill in the regions beneath the curve in multiple colours. For example, if your data is

 x = -5:0.01:5;
 y = exp(x);

you can plot the curve as

 plot(x,y);

Use the find command to determine the last index of y that has a value less than or equal to 5. (In this case, that index is 661.) Then, colour in the area beneath the curve in green for all those y values less than or equal to 5

 hold on;
 area(x(1:661),y(1:661),'basevalue',0,'FaceColor','g');

Now colour in the area beneath the curve in red for all those y values greater than 5

 area(x(662:end),y(662:end),'basevalue',0,'FaceColor','r');

In the command window, type help area for more details on this command (and the use of basevalue).

[Mark]

This is great but I'm having some trouble finding the last index of y that is less than or equal to 5. When you say use find do you mean, find(x_1<=5)?

[Geoff Hayes]

Almost. In the above example, I would use y and do something like

 yIdcs = find(y<=5);

to get all indices in y whose value is less than or equal to 5. Since the exponential function (in the above example) is increasing, yIdcs(end) is the last index (661) that has a value less than or equal to 5.

[Mark]

Thanks again, I still couldn't get the find command to work so I just went into the variable browser and found the indices manually, works like a charm now