Usually, when the "Cannot read property 'xAxis' of undefined" error is displayed, it's because your Salesforce report is not Grouped by any field.
The key 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.
Our guide to configuring the Salesforce widget article explains how Grouping and Summarizing works and how your reports translate and plot within Geckoboard. Follow the steps there and create X and Y-Axes. Once that's done, your report should display just fine.