How can we see a full line instead of a dot in line + bar chart, given only one value in line axis

If I understand your question correctly, it seems that you are wanting to have the line portion of the line + bar chart be the same value for all entries on the x axis, even though there is only one value in the data.

Perhaps one of the following could be helpful:

1.) Add records to the dataset so that the value needed is attached to each x-axis value.

2.) If the value is static, use either the Goal option for the metric being used or a line using the Scale Marker option.

3.) Create a beast mode calculation to return the desired value for each row of the data.