Excel now has data bars which allow for quick visualization within cells but sometimes it is fun to hack your own. JuiceAnalytics walks through how to build in-cell bar graphs for lightweight data exploration:
The bar graphs are built using the Excel REPT function which lets you repeat text a certain number of times… For in-cell bar charts, the trick is to repeat a single bar “|”. When formatted in 8 point Arial font, single bars look like bar graphs.
Their follow-up post has more ideas like negative numbers and formatting improvements.