• Grouping by using shape or color. If the grouping variable is categorical, we can use shape or color or both. If the grouping variable is continuous, usually we use color or size.
  – Example: Scatterplot between Age and Height grouped by Sex or filled by weight.
• Dealing with overplotting.
  –  Make the points semitransparent or use colors.
  –  Example: Scatterplot between Carat and Price of Diamond.

– Use jittering or box plots.
– Example: Scatterplot between time and chicken weight.

• For illustration purposes, you can transform a continuous variable into a categorical variable.
  – ex: Age can become age groups: 18 to 24, 35, to 44, etc.
• Ordinal variables can sometimes be treated as continuous if there are enough categories
  – In social sciences, Likert-type scale is usually used to measure people’s perceptions,
  attitude, etc. and could be treated as continuous.

Reference

• R Graphics and cookbook