The number of pixels used on vector plots can be changed via the ``change number of pixels'' option in the v)ector menu. This controls the number and average size of the arrows which appear (i.e., one arrow is plotted at the centre of each pixel).
The size of the arrows on vector plots is proportional to the magnitude of the vector quantity at that pixel, where the maximum size is set from the maximum plot limit for the x, y and z components of the vector quantity being plotted such that the longest arrow fills one pixel. These limits can be changed manually via the l)imits menu options. Where these limits are nowhere near the actual values of the vector field, arrows can appear either very big (just a line across the screen) or extremely small (appearing as just dots). Pressing `w' in interactive mode automatically adjusts the arrows to sensible proportions (this is the equivalent of pressing `a' for non-vector quantities). Alternatively pressing `v' (to decrease) or `V' (to increase) can be used to adjust the arrow lengths (the change can be multiplied by 10 or more by first pressing `z' one or more times before pressing 'v' or 'V').
Vector arrows are by default plotted using the current foreground colour index (i.e., as used for plotting the axes). To plot in the background colour index instead set the ``use background colour for arrows'' option in the v) menu.
The legend which appears on vector plots can be turned on or off via the ``vector plot legend settings'' option in the v) menu.
The position of the vector plot legend can be set either interactively by positioning the mouse and pressing 'H' or manually via the ``vector plot legend settings'' option in the v) menu.
To plot a vector plot that uses stream/fieldlines instead of arrows, set the ``plot stream/field lines instead of arrows'' option in the v) menu. This option performs a simple integration of the interpolated vector field to get the stream function, the contours of which are then plotted (note that the number of contours can be changed via the ``change number of contours'' option in the r)ender menu). It is generally advantageous to use a larger number of pixels for the vector interpolation (See §??) to get smooth contours.
At present this option works quite well for smooth vector fields but can perform poorly for vector fields with strong gradients.
Vector plots can be plotted using arrows without heads using the ``turn arrow heads on/off'' option in the v)ector plot options menu.
On rendered plots often arrows can appear where there are apparently no SPH particles because the interpolation is performed to all pixels within $2h$ of an SPH particle. Such arrows in regions of few or no particles can be hidden using the ``hide arrows where there are no particles'' option in the v) menu. A threshold number of particles for each pixel can be specified, below which no arrow will be plotted on that pixel.
Vector plots are either in a cross section slice or are column integrated projections depending on the setting of the ``switch between cross section/projection'' option in the x) menu. Setting this to cross section and plotting a vector plot produces a vector plot in a cross section slice.
An option to plot all vector arrows of the same length (instead of the default option where the length of the arrow is proportional to the vector magnitude) can be set from the v) menu.