Hello,
you need to be in Edit mode of the chart.
- Select the chart (green squares visible)
-
Right click -> Edit – or – Double click into the chart (grey border appears)
Now you can hover over the data points and get some values.

[Update 1] (as per request in comment)
Tested using LibreOffice:
Version: 6.4.4.2, Build ID: 3d775be2011f3886db32dfd395a6a6d1ca2630ff
CPU threads: 8; OS: Linux 4.12; UI render: default; VCL: kf5;
Locale: en-US (en_US.UTF-8); UI-Language: en-US, Calc: threaded
Version: 6.3.6.2, 074d8a661f4036fca8fa0cbfa33a497
CPU threads: 8; OS: Linux 4.12; UI render: default; VCL: kde5;
Locale: en-US (en_US.UTF-8); UI-Language: en-US, Calc: threaded
Version: 6.2.8.2, Build ID: f82ddfca21ebc1e222a662a32b25c0c9d20169ee
CPU threads: 8; OS: Linux 4.12; UI render: default; VCL: kde5;
Locale: en-US (en_US.UTF-8); UI-Language: en-US, Calc: threaded
Version: 5.4.7.2, Build ID: c838ef25c16710f8838b1faec480ebba495259d0
CPU threads: 8; OS: Linux 4.12; UI render: default; VCL: kde4;
Locale: en-US (en_US.UTF-8); Calc: group
[Update 2] This doesn’t work on any of my GNOME/GTK3 based test systems (Ubuntu 18.04 / 20.04 , Fedora 32) and seems to be a bug.
Hope that helps.
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