As an auxiliary line. Randomly select an endpoint on the line you want to draw diagonal shadow, draw any line to make a pattern, and connect it to another endpoint to close it. Rectangular patterns such as rectangles and squares can be used;

This is also an "H" filling. Select the fill style (shadow) you want, and click Step 1 Rectangular Fill;

Delete the guide. Delete the auxiliary line "e" made in step 1, leaving only the straight line you want, and the rest is the shadow under the straight line and the line.