Probably easier to do using worksheet functions than chart trendlines.
basic approach:

Start with the equation for a straight line y=mx+b.
We want to constrain the equation so that the point (x0,y0) are on the
line. Substitute into the equation: y0=m*x0+b
solve for b: b=y0-m*x0
substitute b into original equation: y=mx+y0-m*x0
Associate like terms: y-y0=m*(x-x0)+0 which, if we define new variables
Y=y-y0 and X=x-x0 looks like Y=m*X.
We can now obtain m using the SLOPE function on columns containing y-y0
and x-x0. b is obtained from the above expression for b (b=y0-m*x0).


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MrShorty
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