One of the great powers of working with statistics is the ability to work with and even quantify uncertainty.
This is not always a familiar concept for lay viewers, so visualizing this presents a challenge. Marks on paper imply specificity, so care must be taken when representing a range.
Uncertainty may be expressed as an interval or range of values that reasonably represent the result. Sometimes it is expressed as a range of possible outcomes or paths.
Some ways to represent uncertainty include error bars, fading gradients, violin plots or even dotted lines.
Clear annotation is also key to clarifying what is being represented by the visualization.