Know Your Audience, Know Your Data and Choose Wisely - The Who, The What, and The How
Before finalizing a graph, you should always analyse your users and data and then choose the best way of represent the data to your users.
Answering you question, bar graphs can be designed for mobile in two ways:
- Fitting the bars within the screen - This option is preferred if you have only a limited number of bars.
- Having a horizontal scroll for the bars - This option is preferred if you have a large number of bars.
Also, using responsive bar graphs we can design the graph for mobile to show all the bars within the small screen but, this would reduce the width of the bars considerably if you have a large number of bars. In such cases, it would be a good choice to have a horizontal scroll for the bars.
Reference: Data Visualization Advice for UX Designers