I'm plotting a tangent line to a region and, depending when I evaluate it, I get inconsistent results. I want to use the first way of doing things for flexibility but it doesn't seem to be working. What is going wrong?

The set up code is:

mu1 = -1.0; mu2 = 1.0; ss1 = 1; ss2 = 4; cov = 1;
bivModel = 
 MultinormalDistribution[{mu1, mu2}, {{ss1, cov}, {cov, ss2}}]
myFun[x_, y_] := PDF[bivModel, {x, y}];
a = -2.7; b = 1.6;
yVec = D[myFun[x, y], y];
xVec = D[myFun[x, y], x]

Then when I create a graph:

Show[RegionPlot[
  PDF[bivModel, {x, y}] >= contourval, {x, -7, 3}, {y, -4, 7}], 
 Plot[-(xVec //. {x -> a, y -> b})/(yVec //. {x -> a, y -> b})*(x - 
      a) + b, {x, -7, 3}], Graphics[Point[{a, b}]]]

I get this weird thing:

enter image description here

But then if I perform the simplification that I expect to occur within the expression I get what I am expecting. Typing:

-(xVec //. {x -> a, y -> b})/(yVec //. {x -> a, y -> b})*(x - 
          a) + b

gives

1.6 + 3.21739 (2.7 + x)

And substituting this back in the above gives:

Show[RegionPlot[
  PDF[bivModel, {x, y}] >= contourval, {x, -7, 3}, {y, -4, 7}], 
 Plot[1.6` + 3.2173913043478266` (2.7` + x), {x, -7, 3}], 
 Graphics[Point[{a, b}]]]

Running this gives the following figure that I was expecting (and which is correct).

Why is the first expression (using //.) not working?

enter image description here

  • 2
    Because of how Plot evaluates its arguments. Try wrapping the first argument with Evaluate – mmeent Dec 5 at 16:47
  • This works, but I have a more complicated instance where it does not seem to be working, so will edit my original question to include it. – GerardF123 Dec 5 at 17:04

The use of "Evaluate" suggested by mmeent above works. I also realized that instead of using RepeatReplace, I could simply turn xVec and yVec into functions. Once these are substituted in

Plot[-(xVec[a,b])/(yVec[a,b])*(x - 
      a) + b, {x, -7, 3}], Graphics[Point[{a, b}]]]

the issue seems to disappear.

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