Y axis data point alignment with x axis dates

[Gary Pierce]

I have a graph with a Y axis that plots a population number ending each month
for approximately two years. Each month the graph grows with the addition of
a new month's data. The X axis evenly and correctly displays each month
across the bottom of the graph. The problem is that many of the data points
on the Y axis do not align with the corresponding month and fall between
months or the ticks along the bottom of the graph area. Some are on the
mark, some are a little off, and a few are way off. How do I get the data
points to aligh with the month of the X axis along the bottom of the graph so
that the trendline and display is not skewed?