Regarding your first question about mode. You can use some features of the table to find the mode. One way to do this is to create a column that calculates a count of the cases and place this at a higher hierarchical level, so at first it will just count all of the cases. Then move whatever attribute you want up to that level to group by each unique value. Then sort the count descending. This will show you how often each value is present starting with the most frequent. See this video and this shared doc. Using this technique you can see if there is more than one mode.
Regarding bar graphs, it is typical that each bar in a bar graph presents some categorical value rather than a numerical value, so we follow that pattern in the expectations for the types of attributes that can be used to generate bar and line graphs.