The ornamental design for a heightgauge, as shown and described.
1. Height gauge
1.1 : Perspective
1.2 : Perspective
1.3 : Perspective
1.4 : Front
1.5 : Back
1.6 : Right
1.7 : Left
1.8 : Top
1.9 : Bottom
The broken lines showing of control box, command wheel, and feeler is included for the purpose of illustrating portions of the “article” and forms no part of the claimed design.