ASME Y14.5-2009 ¶ 1.7.2 "Extension lines are used to indicate the extension of a surface or point to a location preferably outside the part
outline. See para. 1.7.8. On 2D orthographic drawings, extension lines start with a short visible gap from the outline of the part and extend beyond the outermost related dimension line. See Fig. 1-8. Extension lines are drawn perpendicular to dimension lines. Where space is limited, extension lines may be drawn at an oblique angle to clearly illustrate where they apply. Where oblique lines are used, the dimension lines are shown in the direction in which they apply. See Fig. 1-10."
While the examples given only show horizontal or vertical dimensions, the text does not limit it to those. Since extension lines at odd angles are often used depending on the feature they are defining, oblique dimensions at an angle are allowable under the standard (as long as the dimension line is in the direction in which it applies).
Call it an extension of principal. ;-)
“Know the rules well, so you can break them effectively.”
-Dalai Lama XIV