For you first issue, you could import the data as a string and then format then in a after import process to the data you want to use. Also, if you use the Excel Importer from the App Store, you can run a microflow for the related column where you can add a specific handling of your attribute.
For your second issue, it depends on your personal favorite for the UI. But what you can do is, that you build a helper object structure where you build the objects for the line chart as you need them and then aggreagte them and provide them to the line chart in a Data Source Microflow.
I am able to display correctly processor Time Series. But Memory Time series are showing values even for empty dates.