Cannot read property 'xAxis' of undefined error occurs because your Salesforce report is not grouped by any field.
The solution here would be to group by a field within your report. If needed, you can then also summarize by another field. Grouped fields act as the X-Axis and summarized fields act as Y-Axis within Geckoboard. If you edit the report in Salesforce, you'll be able to Group and Summarize fields.
For more information on how grouping and summarizing works and how your reports translate and plot within Geckoboard, see How Salesforce reports connect with Geckoboard.