When working with filled contour plots in Julia using the Plots.jl package, you may encounter the need to create plots without contour lines and with smooth interpolation using the GR backend. In this article, we will explore three different ways to achieve this.

## Option 1: Using the `contourf` function

The first option is to use the `contourf` function provided by the Plots.jl package. This function allows us to create filled contour plots with smooth interpolation and without contour lines.

```
using Plots
# Generate some data
x = range(-5, stop=5, length=100)
y = range(-5, stop=5, length=100)
z = [sin(x[i] + y[j]) for i in 1:length(x), j in 1:length(y)]
# Create the filled contour plot
contourf(x, y, z, fill=true, levels=100, color=:viridis, linecolor=:none, interpolation=:bilinear)
# Customize the plot
title!("Filled Contour Plot")
xlabel!("X")
ylabel!("Y")
```

This code snippet generates some sample data and creates a filled contour plot using the `contourf` function. The `fill=true` argument ensures that the plot is filled, while the `linecolor=:none` argument removes the contour lines. The `interpolation=:bilinear` argument specifies smooth interpolation.

## Option 2: Using the `contour` function with `fill=true`

Another option is to use the `contour` function with the `fill=true` argument. This allows us to create filled contour plots without contour lines and with smooth interpolation using the GR backend.

```
using Plots
# Generate some data
x = range(-5, stop=5, length=100)
y = range(-5, stop=5, length=100)
z = [sin(x[i] + y[j]) for i in 1:length(x), j in 1:length(y)]
# Create the filled contour plot
contour(x, y, z, fill=true, levels=100, color=:viridis, linecolor=:none, interpolation=:bilinear)
# Customize the plot
title!("Filled Contour Plot")
xlabel!("X")
ylabel!("Y")
```

This code snippet is similar to the previous one, but it uses the `contour` function instead of `contourf`. The `fill=true` argument ensures that the plot is filled, while the `linecolor=:none` argument removes the contour lines. The `interpolation=:bilinear` argument specifies smooth interpolation.

## Option 3: Using the `heatmap` function

The third option is to use the `heatmap` function provided by the Plots.jl package. This function allows us to create filled contour plots without contour lines and with smooth interpolation using the GR backend.

```
using Plots
# Generate some data
x = range(-5, stop=5, length=100)
y = range(-5, stop=5, length=100)
z = [sin(x[i] + y[j]) for i in 1:length(x), j in 1:length(y)]
# Create the filled contour plot
heatmap(x, y, z, fill=true, color=:viridis, interpolation=:bilinear)
# Customize the plot
title!("Filled Contour Plot")
xlabel!("X")
ylabel!("Y")
```

This code snippet uses the `heatmap` function to create a filled contour plot. The `fill=true` argument ensures that the plot is filled, while the `interpolation=:bilinear` argument specifies smooth interpolation.

After exploring these three options, it is clear that the first option, using the `contourf` function, is the most straightforward and concise way to create filled contour plots without contour lines and with smooth interpolation using the GR backend in Julia. It provides all the necessary functionality and customization options in a single function call.