refactor Makefile with variables

Makefile

+BIN = "~/.local/bin"
+VENV = "~/.local/share/venvs"
+
 .PHONY: install uninstall venv
 
 install:
-	cp src/main.py ~/.local/bin/gplt
-	chmod +x ~/.local/bin/gplt
-	cp src/plot.py ~/.local/bin/
-	cp src/multi_bar.py ~/.local/bin/
-	cp src/error.py ~/.local/bin/
-	cp src/log.py ~/.local/bin/
-	./scripts/set-version.sh ~/.local/bin
+	mkdir -p "${BIN}"
+	cp src/main.py "${BIN}/gplt"
+	chmod +x "${BIN}/gplt"
+	cp src/plot.py "${BIN}/"
+	cp src/multi_bar.py "${BIN}/"
+	cp src/error.py "${BIN}/"
+	cp src/log.py "${BIN}/"
+	./scripts/set-version.sh "${BIN}"
 
 uninstall:
-	rm ~/.local/bin/gplt
-	rm ~/.local/bin/plot.py
-	rm ~/.local/bin/multi_bar.py
-	rm ~/.local/bin/error.py
-	rm ~/.local/bin/log.py
+	rm "${BIN}/gplt"
+	rm "${BIN}/plot.py"
+	rm "${BIN}/multi_bar.py"
+	rm "${BIN}/error.py"
+	rm "${BIN}/log.py"
 
 venv:
-	./scripts/create-venv.sh ~/.local/share/venvs
+	./scripts/create-venv.sh "${VENV}"