Eliminate spurious messages

Fixes #288

visualize.Rmd

@@ -300,7 +300,7 @@ If you prefer to not facet in the rows or columns dimension, use a `.` instead o
 
 How are these two plots similar? 
 
-```{r echo = FALSE, out.width = "50%", fig.align="default"}
+```{r echo = FALSE, out.width = "50%", fig.align="default", message = FALSE}
 ggplot(data = mpg) + 
   geom_point(mapping = aes(x = displ, y = hwy))
 
@@ -335,7 +335,7 @@ Here `geom_smooth()` separates the cars into three lines based on their `drv` va
 
 If this sounds strange, we can make it more clear by overlaying the lines on top of the raw data and then coloring everything according to `drv`. 
 
-```{r echo = FALSE}
+```{r echo = FALSE, message = FALSE}
 ggplot(data = mpg, mapping = aes(x = displ, y = hwy, color = drv)) + 
   geom_point() +
   geom_smooth(mapping = aes(linetype = drv))
@@ -354,7 +354,7 @@ knitr::include_graphics("images/visualization-geoms-4.png")
 
 Many geoms, like `geom_smooth()`, use a single geometric object to display multiple rows of data. For these geoms, you can set the `group` aesthetic to a categorical variable to draw multiple objects. ggplot2 will draw a separate object for each unique value of the grouping variable. In practice, ggplot2 will automatically group the data for these geoms whenever you map an aesthetic to a discrete variable (as in the `linetype` example). It is convenient to rely on this feature because the group aesthetic by itself does not add a legend or distinguishing features to the geoms.
 
-```{r, fig.width = 3, fig.align = 'default', out.width = "33%"}
+```{r, fig.width = 3, fig.align = 'default', out.width = "33%", message = FALSE}
 ggplot(data = mpg) +
   geom_smooth(mapping = aes(x = displ, y = hwy))
               
@@ -435,7 +435,7 @@ ggplot(data = mpg, mapping = aes(x = displ, y = hwy)) +
 
 1.  Recreate the R code necessary to generate the following graphs.
     
-    ```{r echo = FALSE, fig.width = 3, out.width = "50%", fig.align = "default"}
+    ```{r echo = FALSE, fig.width = 3, out.width = "50%", fig.align = "default", message = FALSE}
     ggplot(data = mpg, mapping = aes(x = displ, y = hwy)) + 
       geom_point() + 
       geom_smooth(aes(group = drv), se = FALSE)