Hi Mikkel
I'm not quite sure whether I have understood how much of a mathematical function you would prefer for the graph to be.
But if it should just be a practical solution to the problem in a parametrical, piecewise way. I think this function (attached as m-file). Feel free to use it without any credits, copyright or whatever - and change everything you care to change :D
% Example data roughly taken from your question
x = 0:2500;
cutoff = 1250;
maxwidth = 1250;
minwidth = 100;
And here is the function in clear text (same as attached)
function y = triangularplot(x,maxwidth,minwidth,cutoff)
% TRIANGULARPLOT
% Creates a triangular plot in response to MATLAB Central question 519827
%
% I/O
% y = triangularplot(x,maxwidth,minwidth,cutoff);
%
% INPUT
% x = the x-data to use for the calculation
% maxwidth = the width at x = 0
% minwidth = the minimum width of the channel
% cutoff = the x value at which the width reaches the minimum
%
% 2020-04-2020 Johannes Hougaard <xjhh@novonordisk.com>
% Preallocation of y
y = nan(size(x));
% Calculating the slope for the linear section of the graph
slope = (minwidth-maxwidth)/cutoff;
% Calculating the y-values for x <= cutoff
y(x <= cutoff) = maxwidth + x(x <= cutoff).*slope;
% Setting the y-values aboce cutoff as the minimum width
y(x > cutoff) = minwidth;
% Plotting the result
figure;
plot(x,y,'k-');
set(gca,'NextPlot','add');
plot(x,-y,'k-');
set(gca,'NextPlot','replace');
ylabel('Width (km)');
