The error of format among the given options is "excessive expansion of a visual, either horizontally or vertically".
This refers to stretching or compressing the visual beyond a reasonable scale on the X or Y axis, which can distort the representation of data. The other options listed are potential errors in formatting as well, including failure to use zero at the beginning of a series, distracting use of grids and shading, inconsistent intervals between data points on the X- and Y-axes, and unethical or inappropriate use of visual elements.