Since you mention Excel, something you could do is use Gnumeric to import your .xls files into it, and from there export them as SVG. Unlike Excel, Gnumeric has more export options that would allow you to create more complex elements without having to actually draw them on Inkscape.
There is also an extension for Inkscape called NiceCharts that is good for creating basic pie and bar charts. It hasn't been updated since 2012, but it seems to be working nicely.
Other options are Gnuplot (a command line - with several frontends - driven interactive data and function plotting tool has SVG output for graphs & charts) and Matplotlib (a python plotting library and can also output SVG). Finally, SVG charter is a perl script dedicated to generate SVG charts on web servers.
For diagrams, Graphviz can produce structural information as diagrams of abstract graphs and networks in SVG and other vector formats.