diff --git a/ddresc_visu/draw.py b/ddresc_visu/draw.py
index f29914d..e664dea 100644
--- a/ddresc_visu/draw.py
+++ b/ddresc_visu/draw.py
@@ -68,3 +68,15 @@ class Visualisation:
 	def reload(self):
 		self.mapfile.load()
 		self.draw()
+	
+	def run(self, *, refresh=2):
+		pygame.time.set_timer(pygame.USEREVENT, refresh * 1000)
+		self.draw()
+		while True:
+			ev = pygame.event.wait()
+			if ev.type == pygame.QUIT or ev.type == pygame.KEYDOWN and ev.key == pygame.K_q:
+				return
+			if ev.type in [pygame.VIDEOEXPOSE, pygame.VIDEORESIZE, pygame.USEREVENT]:
+				self.draw()
+
+
diff --git a/ddresc_visu/mapfile.py b/ddresc_visu/mapfile.py
index e940384..e82efa5 100644
--- a/ddresc_visu/mapfile.py
+++ b/ddresc_visu/mapfile.py
@@ -31,17 +31,16 @@ def read_mapfile(fn):
 class MapFile:
 	def __init__(self, filename, start=None, end=None):
 		self.mapfilename = filename
-		self.start = start
+		self.start = start if start is not None else 0
 		self.end = end
 	
 	def load(self):
 		self.status, self.blocks, self.comments = read_mapfile(self.mapfilename)
-		if self.start is not None or self.end is not None:
+		if self.end is not None: # FIXME: what about only start being set?
 			self.filter_data()
 		self.size = max(map(lambda blk: blk[0]+blk[1], self.blocks))
 	
 	def filter_data(self):
-		assert self.start is not None and self.end is not None
 		self.blocks = self.filter_blocks(self.blocks, self.start, self.end)
 		self.blocks = self.trim_blocks(self.blocks, self.start, self.end)