Implement visualization for ATR

I can't believe this was the first multi-input indicator I added.

README.md

@@ -80,7 +80,7 @@ golden_cross_chart |> visualize |> lwc_show
 - [ ] ALMA
 - [ ] AO
 - [ ] Aroon
-- [ ] ATR
+- [ ] [ATR](https://www.tradingview.com/support/solutions/43000501823-average-true-range-atr/)
 - [x] BB
 - [ ] BOP
 - [ ] CCI

src/ATR.jl

@@ -0,0 +1,23 @@
+denominated_price(atr::ATR) = false
+
+"""$(TYPEDSIGNATURES)
+
+Visualize ATR using 1 lwc_line.
+"""
+function visualize(atr::ATR, opts::Union{AbstractDict,Nothing}, df::DataFrame)
+    name = indicator_fields(atr)[1]
+    kwargs = Dict(
+        :label_name => "ATR $(atr.period)",
+        :line_color => "#1B9AAA",
+        :line_width => 3
+    )
+    if opts !== nothing
+        merge!(kwargs, opts)
+    end
+    atr = replace_missing_with(0, df[!, name])
+    return lwc_line(
+        df.ts,
+        atr;
+        kwargs...
+    )
+end

src/TechnicalIndicatorCharts.jl

@@ -485,6 +485,7 @@ function visualize(chart::Chart;
     )
 end
 
+include("ATR.jl")
 include("BB.jl")
 include("DEMA.jl")
 include("EMA.jl")

test/ATR.jl

@@ -0,0 +1,18 @@
+using TechnicalIndicatorCharts
+
+@testset "ATR" begin
+    if !isdefined(Main, :sample_candles)
+        include("helper/main.jl")
+    end
+    chart = Chart(
+        "TEST", Minute(1),
+        indicators=[ATR{OHLCV{DateTime,Float64,Float64}}(;period=5)],
+        visuals=[nothing]
+    )
+    candles = sample_candles()
+    for c in candles
+        update!(chart, c)
+    end
+    @test visualize(chart) isa Any
+    # If it gets this far, we can instantiate, update, and visualize.
+end