The task is to plot a set of geographical points (longitude/latitude values) in a map, using a simple xy scatter chart. The problem of using regular maps as background images is that longitudinal and latitudinal lines are not straight (http://www.learner.org) and xy scatter charts use straight lines.
There is already some help out there:
Use a map in an excel chart (https://www.get-digital-help.com): this post adds a background image to an Excel chart and displays the location with xy values. However, the xy values have to "fit" to the map and are not allowed to be pairs of longitude/latitude values.
Create a Map Chart (https://support.office.com): the in-house map in Excel " [...] can only plot high-level geographic details, so cities, latitude and longitudes, and street address mapping isn’t supported."
I was not able to find an open-source map with straight longitudinal and latitudinal lines and decided to create a new map.
How to create a map with straight longitudinal and latitudinal lines?
I wanted to create a map with all countries of the European Union. I found geographical data for all NUTS countries on http://nuts.geovocab.org/, including the polygon(s) for each country. The polygons describe the geographical boundaries or borders of each country. Each polygon or linear ring can be displayed, e.g. with the xy line scatter chart.
For example: below you find an OpenStreetMap version of Malta and the description of Malta with 2 polygons (one for each island, http://nuts.geovocab.org/id/MT.html).
EU Map / Europe with straight longitudinal and latitudinal lines
I wrote an Excel Macro to read polygon data from http://nuts.geovocab.org/ and to display all polygons in a (xy) line chart (each polygon is a new dataset). It all resulted in an EU map with straight longitudinal and latitudinal lines, that can be used as a background picture in xy charts (x-axis: -30 to 40, y-axis: 25 to 75).
Feel free to write me a message (-> Contact).