You can combine multiple geoms into a single ggplot2 plot using the + operator. To achieve your desired output, you need to specify both geom_point() and geom_line() in the same call.

Here's how you can do it:

p <- ggplot(observedData, aes(x = Data_X, y = Data_Y)) +
 geom_point(aes(colour = Data_Z), size = 2) + 
 scale_colour_gradient(low = "green", high="red") + 
 geom_line(aes(x = Model1_X, y = Model1_Y), colour = "blue")

In this code:

• p <- ggplot(observedData, aes(x = Data_X, y = Data_Y)) sets up the plot with x and y axes.
• geom_point(aes(colour = Data_Z), size = 2) creates a scatterplot of observed data points colored by Data_Z. The size = 2 argument increases the size of the points.
• scale_colour_gradient(low = "green", high="red") sets up a color scale for the points, ranging from green to red.
• geom_line(aes(x = Model1_X, y = Model1_Y), colour = "blue") adds a line plot based on your model data (Model1_X and Model1_Y). The line is blue in this case.

The + operator combines these geoms into a single ggplot2 object.