The ornamental design for a stand for monitor, as shown and described.
1. Stand for monitor
1.1 : Perspective
1.2 : Rear perspective view
1.3 : Front
1.4 : Back
1.5 : Left
1.6 : Right
1.7 : Top
1.8 : Bottom
1.9 : Reference
The broken lines indicate portions of the article and a monitor that form no part of the claimed design; reproduction 1.9 is a reference view showing a state which the subject design is installed on the monitor.