GMT

When I tried

gmt grdvector u.grd v.grd -Q+je -JX10 -W > vector+je.ps
gmt grdvector u.grd v.grd -Q+jc -JX10 -W > vector+jc.ps

both give the same results in which vectors are aligned at their centers.

The vector justification is controlled at source:trunk/src/grdvector.c#L568

justify = PSL_vec_justify (Ctrl->Q.S.v.status);        /* Return justification as 0-2 */
if (justify) {        /* Justify vector at center, or tip [beginning] */
    x_off = justify * 0.5 * (x2 - plot_x);        y_off = justify * 0.5 * (y2 - plot_y);

The variable "justify" is supposed to take the value 0, 1 or 2, but it is declared as "bool" at source:trunk/src/grdvector.c#L320. So, when PSL_vec_justify() returns 2, the value 2 may be cast to bool "true", and be stored in "justify" as "true" (=1).

I think this can be fixed by declaring the variable "justify" as int or unsigned int.