Rotating the symbols of a decorated line does not work ("+aangle")

[yvonnefroehlich]

Description of the problem

Based on the documentation at https://docs.generic-mapping-tools.org/dev/plot.html#s it should be possible to rotate the symbols of a decorated line by appending +aangle:

+aangle
Force symbols at a fixed angle, +an for line-normal, or +ap for line-parallel [Default].

However, so far this does not work for me. For quoted lines, it works to rotate the label by appending +aangle.
Related PyGMT comment GenericMappingTools/pygmt#2564 (comment).

Full script that generated the error

Input data: test_data_deco_line.txt

gmt plot test_data_deco_line.txt -R0/10/-1/1 -JX10c/3c -B -S~d2c:+ss1c+a20 -W1p,black -png deco_lines_rot_gmt

Full error message

No error message occurs.

Actual outcome

Expected outcome

The squares along the line should be rotated by 20 degrees (counter-clockwise from the horizontal); similar to quoted lines:

gmt plot  test_data_deco_line.txt  -R0/10/-1/1 -JX10c/3c -B -Sqd1c:+ltext+a20 -W1p,black -png quot_lines_rot_gmt

System information

• Operating system: Windows-10-10.0.19045-SP0
• GMT version (gmt --version): 6.4.0 as well as 6.5.0_95c3431_2023.06.26

[PaulWessel]

Mystery so far. At the end of gmtsupport_add_decoration we have

	/* Build record with Cartesian (hence col = GMT_Z) "x y angle symbol" since we are using -Jx1i */
	S->data[GMT_X][S->n_rows] = L->x;
	S->data[GMT_Y][S->n_rows] = L->y;
	S->data[GMT_Z][S->n_rows] = gmt_M_to_inch (GMT, G->size);
	S->data[3][S->n_rows] = L->line_angle;	/* Change this in inches internally instead of string */
	S->text[S->n_rows++] = strdup (G->symbol_code);

G->line_angle is 20 as given, but S->data[3][S->n_rows] which is S->data[3][0] for the first point remains 0 after the assignment. How is that possible? When the temp file is built and passed to plot again it has x, y ,size, angle, but angle is zero for all record. Hence the result above.