"Nor Faizah " <a7khawarizmi@yahoo.com> wrote in message <ftn1qi$3ea
$1@fred.mathworks.com>...
> Hello
>
> I have plotted a log log scale graph and would like to find
> the slope for a certain range on the graph. How do I find
> it without calculating it manually (by taking two points on
> the graph and apply the formula m=dy/dx) ?
>
> Many thanks.

When you say, "loglog", I believe you mean that originally there was a
function
y = f(x) ,
but instead of plotting y against x, you have plotted Y = log(y) against X =
log(x). If that is the case, the slope you see on the graph is not the same as
the slope of the original function. They are related according to the equation
dy/dx = (dy/dY)*(dY/dX)*(dX/dx)
= y*dY/dX*(1/x) = (y/x)*M
where M is a slope (derivative) of the curve as it appears on the loglog
graph. This means for example that if you have what appears on the loglog
graph to be a straight line with slope M, the derivative of the original curve,
dy/dx, is related to this M by the above (varying) relation, and that would not
be a straight line in x and y coordinates. On the other hand, if you use the
values listed on loglog axes in determining slope, then that is indeed the
derivative of the original function itself. It all depends on what data you are
making use of, x and y, or X and Y.
See the website:
<http://en.wikipedia.org/wiki/Logarithmic_scale#Loglog_plots>
for a discussion of this subject.
Roger Stafford
