Hi, I have noticed this 2 years ago already, and still observe the same problem after reporting it as a problem:
Selecting a time range does not result in that time range showing the graph.
Whatever range I select, and whatever labels, I only seem to be getting 15 minutes to a couple of hours of data shown in the graph. Selecting 7 days with {app!="somethingnotexisting"} should come up with everything not matching that label I presume. Unfortunately the results are always unpredictable and as such all members of my team have abandoned using loki. Sad 
Here is the config used:
auth_enabled: true
chunk_store_config:
max_look_back_period: 24h
ingester:
chunk_block_size: 262144
chunk_idle_period: 15m
chunk_retain_period: 1m
lifecycler:
ring:
kvstore:
store: inmemory
replication_factor: 1
max_transfer_retries: 0
limits_config:
enforce_metric_name: false
reject_old_samples: true
reject_old_samples_max_age: 24h
schema_config:
configs:
- from: "2018-04-15"
index:
period: 24h
prefix: index_
object_store: filesystem
schema: v9
store: boltdb
server:
http_listen_port: 3100
storage_config:
boltdb:
directory: /data/loki/index
filesystem:
directory: /data/loki/chunks
table_manager:
retention_deletes_enabled: true
retention_period: 24h