set ggplot2 datetime to UTC; fix fit plot

This will fix #99

 - ggplot2 timezone set to UTC for POSIXt plotting
 - match the data type in `add_incidence_fit()` of the predictors and
   the incoming ggplot2 date column. This is necessary because of the
   change in #92 where fit data were changed to Date to give more
   sensible coefficients.
 - update POSIXt test plot that was accidentally saved from the KST time
   zone

R/incidence.R

@@ -254,7 +254,7 @@ incidence.POSIXt <- function(dates, interval = 1L, standard = TRUE, groups = NUL
                              last_date = NULL, ...) {
   ## make sure input can be used
 
-  dots <- check_dots(list(...), names(formals(incidence.Date)))
+  dots  <- check_dots(list(...), names(formals(incidence.Date)))
   dates <- check_dates(dates)
 
   ret <- incidence(as.Date(dates),
@@ -266,8 +266,10 @@ incidence.POSIXt <- function(dates, interval = 1L, standard = TRUE, groups = NUL
                    last_date = last_date,
                    ...)
 
-  f <- if (inherits(dates, "POSIXct")) as.POSIXct else as.POSIXlt
-  ret$dates <- f(ret$dates)
+  ret$dates <- as.POSIXlt(ret$dates)
+  if (inherits(dates, "POSIXct")) {
+    ret$dates <- as.POSIXct(ret$dates)
+  }
   ret
 }
 

R/plot.R

@@ -296,9 +296,10 @@ plot.incidence <- function(x, ..., fit = NULL, stack = is.null(fit),
       out <- out + ggplot2::scale_x_date(breaks      = breaks,
                                          date_breaks = db
                                         )
-    } else if (inherits(x$dates, "POSIXct")) {
+    } else if (inherits(x$dates, "POSIXt")) {
       out <- out + ggplot2::scale_x_datetime(breaks      = breaks,
-                                             date_breaks = db
+                                             date_breaks = db,
+                                             timezone    = "UTC"
                                             )
     } else {
       out <- out + ggplot2::scale_x_continuous(breaks = breaks)
@@ -331,6 +332,18 @@ add_incidence_fit <- function(p, x, col_pal = incidence_pal1){
   }
   df <- get_info(x, "pred")
 
+  # In the case that the incidence object is of the type POSIXt, the data from
+  # the fit object must be presented as POSIXt or ggplot2 will throw a fit.
+
+  if (inherits(p$data$dates, "POSIXt")) {
+    # I add half a day here because any fractional days (0.5) will be thrown out
+    # on conversion and I'm not quite sure why that is
+    df$dates <- as.POSIXlt(df$dates) + 43200 # adding half a day
+    if (inherits(p$data$dates, "POSIXct")) {
+      df$dates <- as.POSIXct(df$dates)
+    } 
+  }
+
   out <- suppressMessages(
     p + ggplot2::geom_line(
       data = df,