Use a stacked bar chart when you have multiple groupings and are interested in the proportions between values in each grouping, as well as each grouping's total
For example, to compare the status of leads by the campaign in a report, and also to compare the totals for each status, set record count as the X-axis, status as the Y-axis, and campaign as the Groupings value. The chart displays a single bar for each status, broken down by campaign, with each campaign shown in a different color.
The proportion of each campaign in each status is easy to compare, as are the totals for each status, but comparing a single campaign’s contribution to different statuses, or to the total, may be difficult.
Use a stacked bar chart when you have multiple groupings and are interested in the proportions between values in each grouping, as well as each grouping's total
For example, to compare the status of leads by the campaign in a report, and also to compare the totals for each status, set record count as the X-axis, status as the Y-axis, and campaign as the Groupings value. The chart displays a single bar for each status, broken down by campaign, with each campaign shown in a different color.
The proportion of each campaign in each status is easy to compare, as are the totals for each status, but comparing a single campaign’s contribution to different statuses, or to the total, may be difficult.
For your reference,
https://help.salesforce.com/articleView?id=chart_bar_stacked_100.htm&type=5
https://help.salesforce.com/articleView?id=chart_bar_grouped.htm&type=5
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