[IMP] charts: disable legend for single dataset

[anhe-odoo]

Task Description

When drawing a chart with only one dataset, we are allowed to choose the position of the legend but no legend will appears even after changing the position. This commit aims to fix thie issue, disabling the select box allowing to change the legend position.

The chart padding has also been adapted to have more padding when there is no title/legend.

Related Task

• Task: 4028903

review checklist

• [ ] feature is organized in plugin, or UI components
• [ ] support of duplicate sheet (deep copy)
• [ ] in model/core: ranges are Range object, and can be adapted (adaptRanges)
• [ ] in model/UI: ranges are strings (to show the user)
• [ ] undo-able commands (uses this.history.update)
• [ ] multiuser-able commands (has inverse commands and transformations where needed)
• [ ] new/updated/removed commands are documented
• [ ] exportable in excel
• [ ] translations (_t("qmsdf %s", abc))
• [ ] unit tested
• [ ] clean commented code
• [ ] track breaking changes
• [ ] doc is rebuild (npm run doc)
• [ ] status is correct in Odoo

[robodoo]