Line Graph

Hi PRAVINV  - create a measure will count sites by Risk_Level while filling in missing data with zeros.

SitesCount =
VAR SelectedYearHalf = SELECTEDVALUE('Data'[AssessmentYear])
VAR SelectedRiskLevel = SELECTEDVALUE('Data'[Risk_Level])
RETURN
CALCULATE(
DISTINCTCOUNT('Data'[Site_Name]),
'Data'[AssessmentYear] = SelectedYearHalf,
'Data'[Risk_Level] = SelectedRiskLevel
) + 0

add this measure in your y-axis .

reg. tooltip, we have tooltip section of your visual, add Site_Name.Ensure the Y-Axis is set to the SitesCount measure, not to Site_Name directly.In the Format Your Visual pane, under Y-Axis, disable Concatenate Labels and Show all items (if applicable).

Hope this works in your case.