ContourPlot[Evaluate@{x + y == 1, x - y == 1}, {x, -2, 2}, {y, -2, 2}]
How can I add the perpendicularity symbol (like the red symbol shown) exactly at the vertex where the two lines meet, to indicate that they are mutually perpendicular?
Playing around with this problem I came up with a couple of variations:
ContourPlot[{x + y == 1, x - y == 1},
{x, -2, 2}, {y, -2, 2},
Epilog -> Translate[Rotate[Line[1/5 {{1, 0},
{1, 1}, {0, 1}}], Pi/4, {0, 0}],
{1, 0}]]
ContourPlot[{x + y == 1, x - y == 1,
ChessboardDistance[{x + y, x - y},
{1, 1}] == 1/5},
{x, -2, 2}, {y, -2, 2},
Exclusions -> None]