First, always put units on your plots. A plot axis should never be without a label and a unit.
Second, consider how the heats of formation are calculated/determined. Are these values that are actually measured experimentally, or are they derived from something else that is determined experimentally? If the latter, it may be helpful to look at those values instead.
Third, are you comparing apples to apples in terms of the substitution point (is the functional group on carbon position 1?) In the case of carboxy-terminated, is the C of the carboxy group part of the chain or in addition to the chain?
Finally, I don't think your numbers are right. Just as a check, I looked up the gas phase values of methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol at the webbook and found, respectively: -205, -234, -256, -277, -298 kJ/mol, which seems like a consistently decreasing trend. Are you sure you recorded the right values? As a side note, 2-propanol (isopropanol) has a heat of formation of -273 kJ/mol, compared to -256 kJ/mol for 1-propanol.