Plot style details

Apart from adding entirely new plot styles, version 6 also makes a few tweaks to existing plot styles.

• More terminals (png/jpeg/sixel/kitty) scale the size of a "dot" by the current line width. Some other terminals already did this.
• Plot styles that draw error bars place a gap at the position of the data point. The size of the gap is controlled by set pointintervalbox.
• It is no longer possible to read in-line data from pseudofile '-' while inside a multiplot. Use a data block instead (see new multiplots).