When drawing chart borders, RainPro defaults to moving 4 points in from the edges to draw the boundary line. This is reduced in proportion to the chart size, but when it appears as a gap between tiles created with the ‘Trellis’ property, it may be excessive for a collection of very small plots. To over-ride this:

chSet ('trellis' 4 5)('gutter' 1)

To have all the small charts abutting, set the gutter to 0.