DynamicModule[{linePosition = 1}, Column[
{
Row[
{
Graphics[{FaceForm[LightGray], EdgeForm[Gray], Rectangle[{0, 0}, {10, 2}],
Dynamic@{Dashed, Line[{{0, linePosition}, {10, linePosition}}]}},
PlotRange -> {{0.0, 10}, {0, 2}}, ImageSize -> {400, 200}],
VerticalSlider[Dynamic[linePosition], {0, 2},
Appearance -> {LeftArrow}, ImageSize -> {Automatic, 80}]
}
]
}]]
