Can't reproduce stat_smooth using `loess` when x-axis is Date

Question

I am using `ggplot2` to get a smoothed estimation of my data ``` ggplot(yy)+geom_smooth(aes(x=Date,y=value),method='loess') ``` It works fine. Now, when try to reproduce this using `loess` function directly, I get an error: ``` loess(value~Date,yy) Error in simpleLoess(y, x, w, span, degree, parametric, drop.square, normalize, : NA/NaN/Inf in foreign function call (arg 2) In addition: Warning message: In simpleLoess(y, x, w, span, degree, parametric, drop.square, normalize, : NAs introduced by coercion ``` I think that `ggplot2` version works because it sets some default parameters. Here my data yy : ``` yy <- data.table::data.table( Date = as.Date(c( "1997-01-01", "1997-02-01", "1997-03-01", "1997-05-01", "1997-07-01", "1997-09-01", "1997-10-01", "1997-12-01", "1998-01-01", "1998-02-01", "1998-04-01", "1998-07-01", "1998-09-01", "2002-04-01", "2004-12-01", "2005-12-01", "2006-12-01", "2007-12-01", "2009-10-01", "1997-06-01", "1997-11-01", "2002-03-01", "2002-07-01", "2002-09-01", "20…

Accepted Answer

You need to convert the `"Date"` class object `Date` to a numeric vector:

```
R> loess(value ~ as.numeric(Date), yy)
Call:
loess(formula = value ~ as.numeric(Date), data = yy)

Number of Observations: 115 
Equivalent Number of Parameters: 4.53 
Residual Standard Error: 66.7
```

That coercion will be going on internally with `ggplot()`'s code - after all, those dates need to be assigned to cartesian coordinates at some point in order to plot them.

Can't reproduce stat_smooth using `loess` when x-axis is Date

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