Interpretation of y-axis in partial dependence plot First off, I know there are many questions on this site similar to this one. I've read them, and have not been able to find a solution.
In Elements of Statistical Learning, the following figure shows partial dependence plots for California Housing Data:

The text defines partial dependence of $f(X)$ on $X_S$ as $f_S(X_S) = E_{X_C}f(X_S, X_C)$, the marginal average of $f$.
I'm wondering how to interpret the y-axis of these plots. Based on the definition, I would expect the y-axis to be the housing price, as the given x-axis variables vary, while accounting for the averages of all other variables. But that can't be the case, because the y-axis has negative values, and all values are in the range -1 to 2.
The scikit-learn documentation shows how to make the plots here: https://scikit-learn.org/stable/auto_examples/inspection/plot_partial_dependence.html#sphx-glr-auto-examples-inspection-plot-partial-dependence-py.
Other questions have asked specifically about the implementation in R for classification, which uses a logit, and explains the negative values. But I'm wondering about the regression case, as described in Elements. 
 A: What we are seeing are changes relative to an overall central tendency. This is easy to miss. The central tendency used is a bit of the author's choice.
In The Elements of
Statistical Learning (2009), the caption of Fig. 10.17 particular mentions: "Partial dependence of median house value on location in California. One unit is \$100,000, at 1990 prices, and the values plotted are relative to the overall median of \$180,000." (emphasis mine). 
Similarly in scikit-learn documentation in the "California Housing data preprocessing" we have the line: y -= y.mean(). This subtracts the mean target value from our target vector and centres our target variables to $0$; as a result our PDPs values will be centred approximately around $0$ too. (sklearn's fetch_california_housing() also states that the target is "in units of 100,000".)
In general, I prefer to show my PDPs uncentred at first instance because it makes it plainly clear how much (approximate) variability a particular feature has on the response but that is a matter of choice. As we see, Hastie et al. (2009) centred around the median, sklearn centred around the mean, while Friedman (2001) "Greedy function approximation: A gradient boosting machine" (Fig. 11) did not centre at all. Similar to Friedman (2001), Greenwell (2017) "pdp: An R Package for Constructing Partial Dependence Plots" also does not centre by default (Fig. 2 and others).
