Categoric variables have values that are labels. e.g. names of plants or types of material. To plot a graph of categoric results, you should choose to do a bar chart.
For example: If you investigate the effect of acid on different metals, e.g. copper, zinc and iron, the type of metal you are using is a categoric variable.
e.g. Claire is investigating the average fuel efficiency of different cars. She creates the following categories: city, small family, family, estate and people carrier. She then finds data from various manufacturers and calculates an average for each category.
A line graph of this data is not very useful because there isn't such a thing as a car which is a bit bigger than a city car, but smaller than a small family car. These categories are also quite arbitrary: it's difficult in some cases to categorise cars. A bar chart is probably most useful in this case as it allows a quick comparison to be made at a glance: