Try this:
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
clear; % Erase all existing variables. Or clearvars if you want.
workspace; % Make sure the workspace panel is showing.
format long g;
format compact;
fontSize = 20;
% Create sample data in the range of -15 to +34
oneLine = linspace(-15, 34, 800);
grayImage = repmat(oneLine, [600, 1]);
subplot(1, 2, 1);
imshow(grayImage, []);
title('The Image', 'fontSize', fontSize);
r = zeros(256, 1);
g = zeros(256, 1);
b = zeros(256, 1);
%Find index of -999
index999 = round((-9.999 - -15)/(34 - -15) * 256)
r(1:index999) = linspace(0, 1, index999);
g(1:index999) = linspace(0, 1, index999);
b(1:index999) = 1;
r(index999:end) = 1;
255-index999
g(index999:end) = linspace(1, 0, 257-index999);
b(index999:end) = linspace(1, 0, 257-index999);
subplot(1, 2, 2);
plot(r, 'r-', 'LineWidth', 9);
hold on;
plot(g, 'g-', 'LineWidth', 6);
plot(b, 'b-', 'LineWidth', 3);
grid on;
xlim([1,256]);
cmap = [r,g,b]; % jet(256);
colormap(cmap);
colorbar;
caxis([-15, 34]);
title('The Colormap', 'fontSize', fontSize);
% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Get rid of tool bar and pulldown menus that are along top of figure.
set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
