Question: From the BodePlot function, I would like to plot the vertical line at horizontal axis equal to 10 as in the magnitude plot below and indicate the coordinate of the intersection point. 

Could anyone help me to do this?

Thank you.

```
BodePlot[1/(1 + s/10 + s^2/100)]

```

[![enter image description here](https://i.stack.imgur.com/Huijt.png)](https://i.stack.imgur.com/Huijt.png)

Answer: 
```
BodePlot[1/(1 + s/10 + s s/100),
 Epilog -> Line[{{1, -100}, {1, .11}}]]

```

or

```
BodePlot[1/(1 + s/10 + s s/100),
 Epilog -> {{Red, Line[{{1, -100}, {1, .11}}], 
            {PointSize[0.05], Point[{1, .11}]}}, 
            {Red, Line[{{1, -90}, {1, -200}}]}}]

```

[![enter image description here](https://i.stack.imgur.com/svP0S.png)](https://i.stack.imgur.com/svP0S.png)

If you want to have the points calculated automatically for your choice of abscissa:

```
mysvalue = 1;
f[s_] := 1/(1 + s/10 + s s/100);
BodePlot[f[s],
 Epilog -> {{Red, 
    Line[{{mysvalue, -100}, {mysvalue, f[mysvalue]}}], 
          {PointSize[0.05], 
    Point[{mysvalue, f[mysvalue]}]}}, 
   {Red, Line[{{mysvalue, -90}, {mysvalue, -200}}]}}]

```

Apparently abscissas are represented on a log scale here.
```
g[s_] = 1/(1 + s/10 + s^2/100);

pt1 = {10, Log10[g[10.]]}

(*  {10, -0.477121}  *)

pt2 = {10, Arg[g[I 10.]]/Degree}

(*  {10, -90.}  *)

BodePlot[1/(1 + s/10 + s^2/100),
 Epilog -> {
   {Red, AbsolutePointSize[4],
    Tooltip[Point[{1, pt1[[2]]}],
     pt1]},
   {Red, AbsolutePointSize[4],
    Tooltip[Point[{1, pt2[[2]]}],
     pt2]}},
 GridLines -> {{{10}, None}, {{10}, None}},
 GridLinesStyle -> Red]

```

[![enter image description here](https://i.stack.imgur.com/O9uDW.png)](https://i.stack.imgur.com/O9uDW.png)