How can I change the dashed line going down to the x-axis? [closed]

Question

Manipulate[
 ContourPlot[r*(y/x), {x, 20, 42}, {y, 20, 42}, 
  ContourStyle -> 
   Directive[RGBColor[1., 0.77, 0.], Opacity[1.], 
    AbsoluteThickness[1.999], Dashed], 
     ContourLabels -> False, 
  Contours -> {27, 28, 28.5, 29.5, 47, 49, 50, 52, 94, 96, 97, 99}, 
     ContourShading -> {Red, Orange, Red, Orange, LightGray, Orange, 
    Red, Orange, LightGray, Orange, Red, Orange, Red}, 
     PlotLabel -> 
   Row[(Style[#1, 
        17] & ) /@ {"\!\(\*\nStyleBox[\"Was\",\n\
FontSlant->\"Italic\",\nFontColor->RGBColor[1, 0, 0]]\)\!\(\*\n\
StyleBox[\" \
      \",\nFontSlant->\"Italic\",\nFontColor->RGBColor[1, 0, 0]]\
\)\!\(\*\nStyleBox[\"I\",\nFontSlant->\"Italic\",\n\
FontColor->RGBColor[1, 0, 0]]\)\!\(\*\nStyleBox[\" \
      \",\nFontSlant->\"Italic\",\nFontColor->RGBColor[1, 0, 0]]\
\)\!\(\*\nStyleBox[\"Speedy\",\nFontSlant->\"Italic\",\n\
FontColor->RGBColor[1, 0, \
      0]]\)\!\(\*\nStyleBox[\"?\",\nFontSlant->\"Italic\",\n\
FontColor->RGBColor[1, 0, 0]]\)  if your speedometer reads ", 
      NumberForm[r, {4, 0}], 
             " mph -> you're actually traveling to  ", 
      NumberForm[r*(y/x), {4, 0}], " mph"}], 
  LabelStyle -> Directive[Black, 20], Axes -> True, Ticks -> True, 

  ImageSize -> 674, AxesOrigin -> {20, 20}, AspectRatio -> 0.65, 
  Epilog -> {{Dashed, Line[{{x, 42}, {x, y}}], 
     Line[{{20, y}, {x, y}}]}, 
         {AbsolutePointSize[32], Inset[car, {x, y}]}}], 

{{x, 30, 
   "old tire diameter inches"}, 20, 42, 0.1, Appearance -> "Labeled"}, 
   {{y, 30, "new tire diameter inches"}, 20, 42, 0.1, 
  Appearance -> "Labeled"}, {{r, 38, "Speedometer reading in mph"}, 
  30, 80, 1, Appearance -> "Labeled"}]

Answer 1

Update: To get a dashed line from the point {x,y} down to the x axis, use

Epilog -> {{Dashed, Line[{{x, 0}, {x, y}}], Line[{{20, y}, {x, y}}]}, 
          {AbsolutePointSize[32], Inset[car, {x, y}]}}]

---

You can change your epilog to:

Epilog -> {{Dashed, Line[{{x, 42}, {x, 20}}], Line[{{20, y}, {x, y}}]},
         {AbsolutePointSize[32], Inset[car, {x, y}]}}]

to get a dashed line from the top to bottom frame.

Or use the options GridLines and Method in ContourPlot: i.e., add

GridLines -> {{{x, Directive[Black, Dashed]}}, None},
Method -> "GridLinesInFront" -> True}

after Epilog->... in your code. You can remove Line[{{x, 42}, {x, y}}] from the Epilog setting if you don't want two overlapping dashed lines.