fix chart step size

apps/block_scout_web/assets/js/lib/coin_balance_history_chart.js

@@ -21,7 +21,7 @@ export function createCoinBalanceHistoryChart (el) {
       var stepSize = 3
 
       if (data.length > 1) {
-        var diff = Math.abs(new Date(data[0].date) - new Date(data[1].date))
+        var diff = Math.abs(new Date(data[data.length - 1].date) - new Date(data[data.length - 2].date))
         var periodInDays = diff / (1000 * 60 * 60 * 24)
 
         stepSize = periodInDays

apps/explorer/lib/explorer/chain.ex

@@ -2997,7 +2997,7 @@ defmodule Explorer.Chain do
     today = Date.to_string(NaiveDateTime.utc_now())
 
     if Enum.count(result) > 0 && !Enum.any?(result, fn map -> map[:date] == today end) do
-      result ++ [%{date: today, value: List.last(result)[:value]}]
+      List.flatten([result | [%{date: today, value: List.last(result)[:value]}]])
     else
       result
     end