How to hide the grid lines from Yaxis and draw only one line from specific Yaxis points?
Refer to HowTos about vertical axis, located within the Community documentation of the Charts module. Here is the relevant part of it:

Refer to #2356053-2: Automatically adjusted value range for vertical axis, which explains how to automatically adjust the minimum and maximum of the Y-axis to fit the available data. Specifying a &quot;Max&quot; and &quot;Min&quot; value is optional. Specifying a minimum of 0 is recommended in the event that numbers will always be positive, while for the maximum value the recommendation is (usually) to just leave it empty.
To experiment with this in your own site, use the column chart with the number of nodes for each content type (located in charts/examples/views ):

If you do not specify a maximum there, you get a close to perfect chart, no?
If however you only have a few nodes (like a dozen or so) but use a huge maximum (like 240 ...), the chart looks not so nice, right?