Since you are using flexibility="3", columns grow or shrink equally as space allows. You need to see the documentation to understand it better. Remove flexibility attribute from all items.
Make the item fluid so that it absorbs any extra space in its
container or shrinks when there is less space. Allowed values are:
auto (columns ...
Unfortunately, the answer from @User6670 is going to give you time zone headaches where the "today" calculation is concerned. Say you have a user in California who accesses your component on May 15, 2020 at 5pm (their time). Since they have a time zone offset of 7 hours, they will see "May 16, 2020" -- the date in UTC.
To fix this, adjust for the user's ...
The following can get you current date
var today = new Date();
var last=new Date(new Date().getFullYear(), 11, 32);
here is a PG https://developer.salesforce.com/docs/component-library/tools/playground/9NLbuG8pA/3/...
This answer doesn't have much to do with Lightning App Builder -- it has more to do with D3 internals and understanding how to manage the value of the SVG's height attribute in LWC.
For a working LWC + D3 starting point, I played around with the D3 example in lwc-recipes to see if I could get the component to adjust its height automatically. I was able to ...