Assuming you don't want to use more complex chips you will indeed need diodes. A simple one like a 1N4148 will do. You will need quite a few of them, but luckily they are cheap.
Note that you will also need current limiting resistors in series with each segment of your display. 1k would be a good guess.
Engineering is not only about creating robust designs, but is about creating a design that meets some specifications. Usually young designers don't fully understand that economic factors are part of the specification. The problem is that sometimes those economic factors are not well specified (that's often a management's fault), but a good designer is somewhat expected to consider also non-strictly-technical aspects in his designs, such as:
BOM-related costs: who cares if 1% of the units fail in the field if it is more economical to ship a new one to the customer instead of making all of them more reliable!
Time to market: who cares if the units are more reliable if our competitors ship their things one month in advance!
Planned obsolescence: (sad, and not environmentally friendly, but usually it goes like this): why should we want to ship units that can last for 20 years if we marketed them to be able to work for 5 (and we made a lower price point for that)?!?
etc.
All this depends on the field to which the design you are creating is targeted, of course. If you aim at a market where a single failure could cost lives (say a new defibrillator), you will apply more safety margins to your design (and you will be forced to do that, in some cases, by mandatory safety standards).
Stricter specs are good if, for example, you are designing a mission-critical board for a space probe for a ~1G$ mission to Pluto. In that case you really would want to foresee the unforeseeable and test for any darned little thing that can go wrong. But this is counterbalanced, economically, by the risk of being sued (or fired) by NASA because your crappy MCU code made all the mission go awry!
To recap, experienced successful designers know how to manage all these economic factors. Of course some of them are really smart and really understand all the delicate balances needed to bring a project to success (be it the new Apple iMostUselessMuchHypedphone or the best instruments for detecting bacteria on a comet). Some others, incredible but true, are just lucky and find the right niche where the "Does the prototype work after being mistreated a bit? Ok! Let's ship it!" mantra works well!
BTW, a good designer should always be wary of requirements he is given. Sometimes people giving you the specs don't really know what they want or need. Even the communication between the designer and the client (or the management) could be misleading. For example, if a client asks for a remotely-controllable barometric station that can work well during winter, it does matter if he is from Alaska or from Saudi Arabia! A good designer should work out the specs with the client, if he is in the position to do so, and a successful designer usually can ask the right questions to nail down the actual specs of the design to make the client happy.
I can understand that for some engineers it is compelling to work out all the details, especially for some passionate individuals that really love creating things that work well. It is not a fault in itself, but it is important to understand that the ability to make tradeoffs is part of engineering. With experience this ability will improve, especially if you work together with good senior designers.
You could also discover that you work for an employer with too low standards for your taste and this could push you to seek another job. But this should be done after you get a bit more experience and learn some tricks of the trade and make you more "appetizing" for a better employer.
Best Answer
As has been implied in comments, if there is a digital 1 or zero event for some lengthy period of time any peak measurement made will need to rely on analogue values being held on a capacitor and no, there is no way of passively achieving this.
I have previously done this by using "peaks detectors" (an opamp with diode and capacitor) for "storing" the most positive voltage and the most negative voltage - two peak detector circuits are needed. Also, some modest amount of low-pass filtering is needed to be done on the raw signal to prevent noise spikes causing too much of an accuracy limit. Here is the general idea for detecting the most positive peak of the signal: -
For my application the detector needed to be done at very high speeds because I was trying to implement a special circuit for detecting and resolving corrupted high speed data. I used a MAX999 comparator for each peak detection circuit.
If your data rate is modest, a better way would be to use a microprocessor and an ADC and actually measure the levels.
Producing a third voltage (or number) that is the mean of the two levels is of course trivial in comparison.